BMC Cancer 2005, 5:33 http://www.biomedcentral.com/1471-2407/5/33 18.1%; CML, 1.6% and 4.2%; and UL, 1.1 and 0.8%, respectively. The standardized rate by age of all leukemias in both countries was higher in boys than in girls, with a more marked male:female ratio in El Salvador than in Mexico City: 1.18 (46.8 per million children:39.5 per million children) vs. 1.01 (58.8 per million children:58.1 per mil- lion children). Whereas standardized rates by age in El Salvador revealed that all types of leukemias were more frequent in boys, this was not observed in AML or CML in Mexico City, where the frequency of both leukemias was higher for girls than that reported for boys (Tables I and II). The low incidence of ALL in youngsters less than 1 year old found in El Salvador is notable, as is the fact that no case of AML in this age group was found in Mexico City. The data for Mexico City, as those for El Salvador, showed that ALL had a peak between the ages of 1–4 years (Table 1 and 2). It is important to emphasize that, for the chil- dren in Mexico City, the age group with the highest AIR after that of the 1–4 year old group was the group of less than one year of age. Among the subtypes of AML, the most frequent in Mexico City was AML M3 (01B9866) with nine cases, giving a fre- quency of 20.9% and an AAIR of 2.21 per million chil- dren. It is noteworthy that all these AML M3 cases were females. Unfortunately, in El Salvador, the frequency of AML M3 could not be assessed, as the major part of AML was classified only in the general category 01B9861. Discussion Because there are only two important reviews that sum- marize the high incidence rates of leukemias in children reported in the world up to 1999 [1,10], it has been very difficult to obtain reliable data on the incidence of cancer in children, including leukemias, in developing countries. In fact, the review of Parkin et al., carried out in 1999, did not include data from either El Salvador or Mexico City because there were no reliable data from these areas at the time (1980 to 1989) at which they performed their study [10]. There are some issues of the present study that must be emphasized. In this study, statistics were not performed to compare the rates in Mexico City with the rates in El Sal- vador over time, because this was not the aim. The inclu- sion of the aforementioned institutions helped guarantee that all children diagnosed with leukemia during the stud- ied period were included. This is due to the fact that, because BB is the only hospital in El Salvador that attends children with leukemia, any child with presumed leukemia is referred to that institution [16]. Similarly, in Mexico City, HP and HG are the only hospitals of the IMSS that attend the children with leukemia whose par- ents have rights to receive medical services from IMSS [17]. Even if there had been an under-registration of cases because some could not be diagnosed (e.g., patients dying before leukemia diagnosis is confirmed), the incidence rates in children of 0–11 years old found in El Salvador are similar to those reported in North America, which range from 36 to 45 http://seer.cancer.gov/csr/1975_2001/, and with rates from South and Central America, which range from 29.0 to 57.9 per million children [10]. In Mexico City, there are various institutions outside the IMSS system that can attend children with cancer, particu- larly leukemia. However, such private institutions repre- sent an economic burden for the family of the patient, in that the treatment for patients with leukemia lasts several years, with the possibility of expenses totaling almost $65,000 USD [22]. Therefore, it is less probable that the family of a child with leukemia would turn down access to the free service offered by HP and HG, opting instead for a costly private institution. For this reason, we think that cases in Mexico would not be underestimated due to patients being treated outside the IMSS system. Mexico is a developing country with a high rate of child mortality in general. The absence of AML cases in infants of less than 1 year in Mexico City is notable because this is the peak age for AML in most developed countries [1,23], and suggests the probability that children younger than 1 year with AML die before being diagnosed. In con- trast to ALL, AML seems to have no worse prognosis in children younger than 1 year [24,25], indicating that all children with AML, regardless of age, run the same risk of not being diagnosed. This increases the possibility of an apparently different pattern of age of AML onset in Mexico City. Little can be argued about the delay of diagnosis in these diseases, due to fast evolution of the disease and short-term mortality when no prompt management is provided; thus, acute leukemias are considered as the ref- erence point for time of diagnosis [26]. As to the validity of measurements, recommended inter- national morphological criteria were used in this study, and all measurements were performed by staff highly spe- cialized in diagnosis and management of children with leukemia [7,16]. This study did not aim to assess incidence trend over time, a parameter that is affected by change(s) in diagnostic cri- teria [27], which were not a variable in this study. Since the period under study was too short to assess the inci- Page 5 of 9 (page number not for citation purposes)