Background: Children who undergo treatment for childhood acute lymphoblastic leukemia (ALL) and lymphoma are at risk for several long-term health problems. Obesity, for which survivors of ALL and lymphoma are also at risk, may further exacerbate these problems. This pilot study evaluates changes in physical activity and body composition among children being treated for ALL and lymphoma and their parents. Procedures: Recently diagnosed adolescent ALL and lymphoma patients were recruited from 2 pediatric hematology and oncology clinics, and matched on age, race, and sex to healthy individuals in the community. Changes in diet, physical activity, and body composition were collected at baseline, 6, and 12 months. Results: All children (n=15) were, on average, 10.3 years of age at enrollment, and were fairly evenly distributed with regard to sex. Analyses revealed a significant difference between cases and controls with respect to the change in body mass index from baseline to 12 months (P=0.01). In addition, controls demonstrated a significantly greater increase in moderate-vigorous physical activity than the cases [229.8 metabolic equivalent of tasks (METs) vs. 23.5 METs]; indicating cases remained fairly inactive over the course of treatment. Conclusions: Our data corroborate previous findings that following treatment for ALL and lymphoma, childhood cancer survivors tend to be less active and at greater risk for obesity than their healthy peers. The present study, which assessed cases prospectively over a 12-month period during the early phases of treatment, extends prior reports by demonstrating that these outcomes are evident at an early stage in treatment. © 2013 Lippincott Williams & Wilkins.