As care for the childhood cancer patient has improved significantly, there is an increasing incidence of treatment-related late effects. Obesity and type 2 diabetes mellitus are common and significant metabolic conditions in some populations of adult survivors of childhood cancer. Results from the Childhood Cancer Survivor Study and other large cohorts of childhood cancer survivors reveal that long-term survivors of acute lymphoblastic leukemia and those who received total body irradiation or abdominal radiotherapy are at highest risk. The potential mechanisms for the observed increase in risk, including alterations in leptin and adiponectin, pancreatic insufficiency, poor dietary habits, sedentary lifestyle, and perhaps changes in the composition of the gut microbiota, are reviewed. Discussion of exercise and diet intervention studies shows that further research about the barriers to a healthy lifestyle and other interventions in childhood cancer survivors is warranted.