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The causes of most leukemias are unknown; only a few exposures have been established as risk factors. Nowadays, the AL in children is considered as the result of the interaction of different environmental factors with the genetic susceptibility to the disease. In the first part of this review, the child's environmental factors were basically covered. Now, in this second part, we are describing some factors of the child himself, as well as other proposed factors, but which have been evaluated in epidemiological research with less frequency. The most important host factor is Down's syndrome. Susceptibility aspects, such as HLA alleles, and aspects related to expositions around the home, late infections and the time in which the expositions occur, require more research. Studies that assess the interaction between the susceptibility to the acute leukemia and the environmental factors could lead to the better understanding of the causal mechanisms of the disease and to more adequate preventive strategies.

Objective: This paper is a revision from recent studies related to acute leukemia (AL) epidemiology in children. Cancer is the second cause of death in the pediatric population in Mexico between 1 to 15 years old. The AL are the types of cancer with more frequency in children below 15 years old and the cost of taking care of a child with cancer represents to the health institutions around 620 thousand dollars per year per case. In different places of the world the frequency of the AL has been increased and in Mexico City this is similar. Data from Instituto Mexicano del Seguro Social, in Mexico City, reports one of the highest worldwide. The environmental risk factors most consistent are the exposure in utero to X-ray, occupational exposure and pesticides. Other risk factors studied, as the exposure to electromagnetic fields, smoking and alcohol consumption in the parents, diet, and others, have had controversial results. These controversies can be explained by methodological mistakes in the studies and also because these studies did not assess the interaction between the susceptibility to AL and the environmental factors, a situation that could lead to the better understanding of the causal mechanisms of the disease. However it is recommended to have an attitude of prudent caution to accept that these factors are a cause of AL, and to have an energetic position in order to decrease the exposure to them and produce more adequate preventive strategies.

Objective: To identify the main age of onset of different malignant tumors in childhood and to describe the distribution of the different tumors in each pediatric age group., Material and Methods: Descriptive survey was used. We reviewed the files of six Mexico City hospitals from 1980 to 1992. We included 4595 cases divided into 13 types of cancer. Peak age was defined when in that year we encountered a frequency equal to or below 10 % of the cases., Results: Peak ages for hepatic, sympathetic nervous system, germ cell tumors, retinoblastoma and rhabdomyosarcoma were between 2 and 3 years of age. Wilms' tumor appeared between the first and fourth years; central nervous system tumors between 4 and 5 years; acute lymphoblastic leukemia between 2 and 4 years; non-Hodgkin's lymphomas between 3 and 6 years; Hodgkin's disease between 4 and 8 years; bone tumors between 10 and 14 years. In acute myeloid leukemia and carcinomas no age peak was found., Conclusions: Lymphomas present an age peak different from that reported in developed countries. In neonates and infants, the most frequent tumor was retinoblastoma.