BMC Cancer 2005, 5:33 http://www.biomedcentral.com/1471-2407/5/33 Table 3: Comparison of annual age-adjusted rates of lymphoid leukemias per million for Mexican and Salvadoran children with those for Hispanic children from three U.S. Cancer Registries and for children from Costa Rica Leukemia type SEER 2001 All races (30) Mexico City, Texas (34) IMSS SEER 1992–1998 Hispanics (29) El Salvador Florida (4) 33.2 44.9 52.0 43.0 34.2* 49.7 California (3) 44.0 Costa Rica (5) 43.1 Lymphoid leukemia IMSS = Instituto Mexicano del Seguro Social. * This rate is only for children 0–11 years old. (References) dence trend of leukemia, no analysis was done in this respect. However, this study evidences once again that choosing only one year to report a disease incidence may result in an over- or underestimation of the incidence of that disease [2]. For example, the observed incidence rate in Mexico City for the year 1996 in this study was much higher than those in the following years, suggesting that this might have been a completely random condition. Although the standardized rates reported in this study were higher than those previously reported for Mexico City, the prior studies [6-8,26,28] were retrospective and therefore are not comparable to the present study. How- ever, the possibility that the incidence of ALL is increasing, particularly in Mexico City as reported in these studies, must be considered [6,8]. This condition has been observed in various areas of Mexico City [8] and is associ- ated with the more polluted areas and with farming areas [6,8]. The elevated incidence of ALL in children of Mexico City agrees with those found in other reports that show that ALL are more frequent in populations of Hispanics [3-5]. Table 3 shows that the AAIR of ALL is the highest reported for the population of Hispanic origin, with the value for the Hispanic populations of Texas being the highest, fol- lowed by that for Florida. Note that the majority of His- panics in Texas are of Mexican origin[4]. The AAIR for ALL reported by SEER for Hispanics for the period 1992–1998 is very similar to that reported for Mexico City and agrees with the fact that, of the Hispanic children included in this report of SEER, 66% come from the Los Angeles register [29], where the great majority of the Hispanic population are of Mexican origin [4]. It is noteworthy that the AAIR of the age peak of ALL in boys in Mexico City is very high (82.4 per million in boys), an is higher than that reported for Costa Rica (76 per million in boys) and by SEER (76.7 per million in both sexs) [5,30]. However, the AAIR reported for Hispan- ics in general in Florida for the 0–4 year-old age group is 87.6 per million, which to our knowledge is the highest AAIR reported in the literature [4]. This finding is impor- tant because this peak in age is related with the higher fre- quency of ALL with the B-cell precursor immunophenotype. Although such data were not reported in this study, this immunophenotype is thought to be more frequent in developed countries [1] and, in addition, is related with genetic rearrangements such as TEL/AML1 that occur in the child during the intrauterine stage [31]. On the other hand, this age peak had been observed in relation to high socioeconomic status and population mixing [1]. The mix of urban and rural populations has been associated with the development of ALL [32,33] and especially with the cases that occurred in children under the age of 5 years [1]. Those states in the U.S.A. that have reported an elevated AAIR for ALL have an elevated level of immigration. The greater portion of Hispanics that emi- grate to these zones come from rural regions. Mexico City has a similar situation, in that it constantly receives an influx of people from rural zones. Although, in this study, patients were determined to be residents of Mexico City, neither the place from which their parents came nor the length of time that the children resided in Mexico City was investigated. It will be important to investigate in the future if the mix between urban and rural populations is related with the very high incidence of ALL in Mexico City and in Hispanics in the U.S.A. Another factor to be considered is the role of pesticides, whether occupational exposure of parents or exposure of children in the home. Also, we must not lose sight of the activities that Hispanic immigrants in the U.S.A. may per- form, such as those related to the cultivation of crops, especially in areas of Texas where a higher frequency of ALL has been reported [34]. In Mexico City, the major zones with highest incidence rate of ALL are the zones in the South of the City where a portion of the population still carries out agricultural activities [8]. In a study carried out in Mexico City, an important relation was found between exposure to pesticides and the risk of developing AL [35]. In an ecological study realized in California, in Page 6 of 9 (page number not for citation purposes)