Today's obesity pandemic began in the United States, spread to Western Europe and other developed regions, and is emerging in developing countries. Its influences on outcomes of childhood cancer are unknown. A recent Children's Oncology Group symposium considered epidemiology of obesity, pharmacology of chemotherapy and outcomes in obese adults with cancer, excess mortality in obese pediatric patients with acute myeloid leukemia (AML), and complications in obese survivors. The salient points are summarized herein. Body mass index (BMI) is the accepted index of weight for height and age. In the US, obesity prevalence (BMI > 95th centile) is increasing in all pediatric age groups and accelerating fastest among black and Hispanic adolescents. Pharmacologic investigations are few and limited: half-life, volume of distribution, and clearance in obese patients vary between drugs. Obese adults with solid tumors generally experience less toxicity, suggesting underdosing. For patients undergoing bone marrow transplantation, obese adults generally experience greater toxicity. In pediatric acute myeloblastic leukemia, obese patients have greater treatment-related mortality (TRM), similar toxicity and relapse rates, and inferior survival compared with patients who are not obese. An excess of female survivors of childhood leukemia who received cranial irradiation are obese. Ongoing treatment effects of childhood cancer may predispose to a sedentary lifestyle. These findings call for measures to prevent obesity, retrospective and prospective studies of chemotherapy pharmacology of analyzed according to BMI and outcomes, additional studies of the obesity impact on outcomes in pediatric cancer, and promotion of a healthy lifestyle among survivors. © 2005 Wiley-Liss, Inc.