To determine whether obesity complicated the treatment of childhood acute lymphoblastic leukemia, we studied the body mass index (BMI) of 63 female when and 51 male patients from the time of diagnosis of acute lymphoblastic leukemia to the time when final height was attained. The BMI z score was calculated for each patient at diagnosis, at end of treatment, and at attainment of final height. Obesity at attainment of final height was defined as a BMI greater than the 85th percentile of the normal reference population. At final height 23 of 51 male (45%) and 30 of 63 female patients (47%) were obese. Girls became obese between diagnosis and the end of chemotherapy (p = 0.02), after which they had no further increase, indicating that chemotherapy may have played a role in their obesity. Boys had a progressive and gradual increase in BMI z score through to attainment of final height. Obesity did not appear to be associated with growth hormone insufficiency, disproportionate growth, or abnormal timing of puberty. We conclude that approximately half the survivors of leukemia in childhood become obese young adults. Many of those treated with the more recent regimens studied are still only in their mid or preteen years and should be advised regarding a more active lifestyle and a healthy diet in an attempt to reduce the incidence of obesity.