Excessive fatness is considered a frequent late complication of treatment for childhood acute lymphoblastic leukemia. Most previous studies, however, were based on body mass index (BMI) rather than more direct fat mass measurements. We studied 95 survivors of childhood acute lymphoblastic leukemia a median of 11 yr (range, 3-23 yr) after diagnosis. BMI values at diagnosis, at cessation of therapy, yearly thereafter for up to 10 yr, and at follow-up were compared with French reference values. Whole body percent fat was measured at follow-up by dual energy x-ray absorptiometry and compared with data from 463 local controls. Adjusted for sex and age, the mean BMI increased significantly during therapy and remained largely unchanged thereafter. At follow-up, BMI did not differ significantly between patients and local controls. On the other hand, the whole body percent fat was significantly increased (mean observed/predicted value, 21.8/19.0%; P < 0.0002). Twenty-five patients (26%) had a percent fat above the 90th percentile of the reference values, which indicates excessive fatness. Adjusted for sex and age, a higher percent fat was related to cranial irradiation or GH insufficiency, but not to sex, the cumulative doses of anthracyclines or corticosteroids, or the type of corticosteroid used. BMI was a poor measure of body fatness.