To confirm an impression that many survivors of acute lymphoblastic leukemia (ALL) are overweight or obese, we retrospectively examined the medical records of 414 patients for height and weight at diagnosis, at completion of treatment, and at annual intervals thereafter. The body mass index, weight/height2, was used as a measure of fatness; population norms for the index were established from 9,003 people between the ages of 1 and 30 years who were examined in a national health survey. The percentile of each patient's index was determined at each observation date. At diagnosis, the study sample was skewed toward leanness; however, at cessation of therapy, the fatness distribution resembled population norms. Statistically significant increases in fatness occurred during the first year off therapy, at the end of which 35% of the children were above the 80th percentile (i.e., overweight) and 12% were above the 95th percentile (i.e., obese). Only 12% were below the 20th percentile. This skewed distribution persisted during the subsequent 4-year follow-up period. Cranial irradiation was associated with a large increase of fatness in one group available for comparisons. Our findings indicate that the first year following cessation of therapy is a time of excessive weight gain among pediatric ALL patients.