Weight gain is common during and following cancer treatment and contributes to many adverse health consequences and increased risk of recurrence for cancer survivors. The purpose of this longitudinal randomized controlled trial was to compare differences in weight change and body composition among newly diagnosed cancer survivors receiving chemotherapy who were randomly assigned to usual care control, aerobic exercise, or resistance exercise interventions. It was hypothesized that cancer survivors become more sedentary during treatment, leading to positive energy balance that in turn worsens body composition. One hundred one subjects completed the 12-month study. Data collected included body composition (body weight and dual energy x-ray absorptiometry scan), aerobic capacity (12-minute walk), and muscle strength (1 repetition maximum). Aerobic exercisers were found to have significant improvements in body composition, aerobic capacity, and muscle strength. Resistance exercisers also benefitted, but were less compliant with the intervention over time. Thus, exercise during and following cancer treatment is an important intervention to maintain and perhaps improve body composition of cancer survivors, which may improve survival, reduce comorbidities, and improve quality of life.