Objective: Scant information exists about the changes in body composition of children during the first months of chemotherapy. These changes can be determined by using a better method than the body mass index. This study compared the changes of body composition by dilution of deuterium oxide in Mexican children with lymphoma and with solid tumors. Methods: Seventeen patients were enrolled and classified as having lymphoma or solid tumor. Body composition was measured by a deuterium dilution technique after the first course of chemotherapy and again after 2 and 6 mo of therapy. Data were compared by means of paired t and Student's t tests. Simple linear regression was applied to examine the relation between age and changes in fat mass (FM) and fat-free mass (FFM). Results: The groups were similar at baseline. Six months after initiation of chemotherapy, weight and height had increased (P < 0.05) in the lymphoma group, whereas only height had increased in the solid-tumor group; total body water, FM, and FFM increased in the lymphoma group (P < 0.01) but not in the solid-tumor group. Age did not influence FM or FFM in either group. Conclusion: In children with lymphoma whose treatment included corticosteroid use, increase in FM content was demonstrated during the first 6 mo of treatment. In patients with solid tumors, FM content did not change during treatment. With an increase in FM content, one should bear in mind that overweight and obesity may result in cardiovascular disease and development of breast cancer in adult life.