Following Iraq’s invasion of Kuwait in August 1990, the United States and other allied nations sent troops to the Persian Gulf region in Operation Desert Shield. In the winter of 1991, the allied forces successfully attacked Iraq in an air campaign and subsequent invasion by ground forces (Operation Desert Storm). Approximately 700,000 American troops participated in these actions. Although casualties were relatively light, thousands of veterans have come forward complaining of various illnesses, including cancer, in the years following the war. During the Gulf War, American troops may have been exposed to several known and potential health risks. These included chemical and biological warfare agents, depleted uranium from munitions, smoke from oil-well fires, infectious diseases, pesticides, petroleum fuels, and vaccines. Some of these substances have been previously associated with different types of cancer through animal laboratory studies and other epidemiological research investigations. For example, combustion products from petroleum include polyaromatic hydrocarbons, benzene, and carbon disulfide, some of which are known to cause lung cancer when inhaled. Exposure to certain pesticides has also been linked to lymphatic and lung cancers. In addition, exposure to radioactive particles has been tied to higher rates of respiratory and other cancers. Information on exposures that took place during the Gulf War, however, has been either incomplete or nonexistent due to the lack of record keeping and measurement before, during, and after the deployment of troops; loss of key records; poor recall by veterans; and other factors. The development of cancer is usually characterized by a long latency period of several years from an initial exposure to a harmful agent to a definitive medical diagnosis. Depending on the nature and extent of the exposure, type of cancer, and characteristics of different individuals, the latency period may be as long as 30 years or more. The most common types of cancers have a latency period of 15 years or more, but in certain situations cancer can develop more quickly. For example, when the immune system is compromised, such as in cases of organ transplants, certain types of cancer may appear within 1 year. Given that there is a lengthy latency period for most tumors, it may be too soon to detect any increase in tumors occurring among Gulf War veterans. Also, since cancer is a relatively rare event, large population groups may need to be observed over several years to assess incidence and determine whether it has changed over time. Furthermore, without credible exposure information, it is hard to form specific hypotheses about what kinds of tumors might occur with what individuals. Although such constraints exist, it is nonetheless important to begin monitoring and assessing whether Gulf War veterans are suffering from an increase in tumors so that appropriate health care and treatment can be provided where needed. With many types of tumors, early detection is important to more effective treatment outcomes. One source for estimating the incidence of cancer among Gulf War veterans utilizes mortality as an indicator for incidence. VA maintains a large administrative database containing records of claims for benefits made by veterans and their dependents. This database, the Beneficiary Identification and Records Locator Subsystem (BIRLS), includes information on more than 40 million individuals. Although not designed as a research database, it does contain information on the vital status of veterans and the location of veterans’ claim forms so that death certificates can be retrieved to ascertain causes of death. VA pays death benefits, including fixed payments for burial and funeral expenses, to eligible survivors of deceased veterans. Dependents are required to submit a copy of the veteran’s death certificate in order to receive these benefits. The BIRLS datafile also contains the veteran’s name, social security number, claim number, current address, and other benefits information. In addition, the file includes a code that indicates whether the veteran was deployed to the Gulf War. VA and other researchers have estimated that death reporting in the BIRLS database is relatively complete in terms of its coverage of the veteran population. Studies using large samples of known deaths have compared vital status reporting in BIRLS with reporting in other national sources of mortality data, such as the Master Beneficiary Record of the Social Security Administration and the National Death Index, and found that BIRLS covers 80 to 90 percent of the deceased veteran population. Another strength of the BIRLS data is that they provide a relatively cost-effective way to assess causes of death among veterans. Obtaining death certificates directly from state health departments involves paying a fee of several dollars for each certificate. Using BIRLS or other mortality databases to assess overall cancer incidence has several key limitations. First, mortality is a lagging indicator of incidence. The latency period for most tumors can be more than 10 years and the period of time until mortality is even longer. Second, in general, mortality is an incomplete measure of cancer incidence because cancer is not always fatal. Mortality data provide good estimates of incidence for cancers that have a high mortality rate (such as lung and liver cancers) but they are less useful for cancers with lower rates of mortality (such as prostate and breast cancers). Because of these limitations, mortality data will systematically underreport overall cancer incidence. In addition, by the time cases show up in the data, it may be too late to help Gulf War veterans. Furthermore, while the reporting of external causes of death, such as accidents, or broad disease categories, such as coronary heart disease, is reasonably reliable, some inconsistencies in cause of death reporting on death certificates have occurred. Death certificates are less accurate in tracking difficult to diagnose diseases, deaths involving multiple causes, and where there are underlying causes of death that are not readily discernible. VA in one published study used data from BIRLS to assess the mortality risk from a range of diseases (including cancer) for all Persian Gulf veterans compared with a sample of veterans who were not deployed to the Persian Gulf. The study covered deaths occurring in a 2-year period after the war (May 1991 to September 1993). Death certificates for those veterans who had been identified as having passed away were obtained from VA regional offices and other locations and were reviewed for cause of death. The study found that overall there was a small but significant excess of deaths among Gulf War veterans (1,765 deaths) compared with nondeployed veterans (1,729 deaths) and that the excess was due mainly to accidents and not disease. Of the 1,765 Gulf War veterans who died during the study period, 119 died from cancer, and there was no statistically significant difference compared with the cancer death rate among nondeployed veterans. If a higher death rate from cancer was expected among Gulf War veterans as a result of some exposure occurring during deployment, then they would be unlikely to appear in this study given the short time period that elapsed. VA is currently updating the study, extending the study period through 1995, and the results should be published later this year. The study’s authors identified another limitation (but it is not clear what effect it had on the study’s findings)—specifically, whether the study’s comparison groups were appropriately matched. Military personnel who were ill or recovering from an illness would not have been deployed to the Gulf War area. However, these personnel were included in the comparison group of nondeployed veterans. This meant that the comparison group may have been somewhat less healthy than the deployed veterans group. The extent to which a higher rate of prior illnesses among nondeployed veterans resulted in a different rate of mortality (or cancer mortality in particular) and thus biased the study findings is unknown. Finally, the use of broad comparisons between deployed and nondeployed veterans rather than more targeted comparisons of veterans based on specific types and levels of exposures may also have affected the soundness of the study. A comparison of deployed and nondeployed veterans has merit for identifying potential widespread and severe health consequences. However, defining the exposed population group as “all those who served in the Gulf War” without regard to individual groups’ exposure histories may obscure some service-connected illnesses. Efforts to exclude from such studies portions of the deployed force who were at low risk of exposure to harmful agents could lead to more meaningful results in such comparative studies. DOD and VA each maintain an automated database containing medical and demographic information on patients discharged from DOD and VA hospitals. DOD’s system collects information from DOD military hospitals. These hospitals are open to active duty personnel and, to a limited extent, retired personnel. VA’s data system, the Patient Treatment File, covers all VA hospitals. VA hospital care is generally available to veterans for service-connected illnesses. Care is also provided on a discretionary basis for nonservice-connected illnesses, depending on the availability of facilities and resources and payment of a required co-payment by the veteran. Veterans who may have been exposed to a toxic substance or environmental hazard while serving in the Gulf War are included in a designated category of veterans who have special eligibility for VA medical care services. Both the DOD and VA data systems include medical discharge diagnoses, which are coded according to standard ICD-9 (International Classification of Diseases, 9th Revision) disease categories. In addition, the data contain relevant information such as social security numbers, date and place of birth, period of military service, length of hospital stay, and surgical and other medical procedures conducted. DOD’s and VA’s hospitalization data systems are large and contain millions of records, but they do not represent the entire active duty and veteran population. While DOD’s data include most hospitalizations of active duty personnel, in large part because DOD medical care is free and readily available to active duty personnel, there have been reports from some veterans’ groups of Gulf War veterans seeking medical care outside DOD. According to these groups, veterans have done so to obtain specialized care or because of concerns that the acknowledgment of their illnesses within DOD could have a negative effect on their military careers. It is not clear whether this would more often be the case for Gulf War veterans with tumors than for nondeployed veterans. VA also has an extensive network of medical centers across the country but the overwhelming majority of veterans use other private and public hospitals.