We investigated obesity [body mass index (BMI) >95th percentile] and being heavy (BMI >85th percentile) in 95 children in first remission more than 2 years after treatment for acute lymphoblastic leukemia or non-Hodgkin lymphoma seen at our institution. Height, weight and BMI at diagnosis, end of treatment and follow-up, and blood pressure at diagnosis were adjusted by z-score for age and sex. At follow-up, obesity and overweight were not more prevalent than in the general population. Median BMI z-scores rose significantly between diagnosis (0.38) and treatment end (0.62) but not during follow-up (0.70). Median weight z-scores rose significantly during both periods (diagnosis 0.23, treatment end 0.49, and follow-up 0.68). Median height z-scores were 0.51, 0.14, and 0.16 for the same 3 time points, respectively. Repeated measures, multivariate logistic regression identified Hispanic ethnicity, younger age at diagnosis, and a positive age:weight interaction as being associated with obesity and being heavy at follow-up. There was no association with diagnosis, sex, age alone, radiation dose or field, metabolic diagnosis in patient/family, height z-score at diagnosis, duration of treatment, and systolic or diastolic blood pressure. Obesity and overweight were a combination of weight gain and height loss during treatment although weight continued to increase after treatment. We did not identify disease-related parameters associated with these effects.