Uncontrolled trials have reported significant weight gain in women with breast cancer during treatment with adjuvant chemotherapy. We prospectively evaluated body composition before (visit 1), immediately after (visit 2), and 6 months after (visit 3) chemotherapy in 20 women with stages I-IIIA breast cancer [body mass index (BMI): 24.1 +/- 3.9 kg/m(2)]. We compared their weight change to 51 age- and BMI-matched healthy controls (BMI: 25.5 +/- 3.8 kg/m(2)). In women with breast cancer, there was no weight change from visit 1-2, or from visit 1-3, but weight increased from visit 2-3 (+1.09 +/- 2.46 kg; P = 0.05). Weight change was not different from controls during either interval. In the breast cancer group, the percentage of body fat assessed by air displacement plethysmography increased, and fat-free mass decreased from visit 1-2 (+2.3 +/- 4% and -2.2 +/- 4%; P = 0.02) and from visit 1-3 (+4.0 +/- 6% and -3.8 +/- 6%; P = 0.01). By dual energy x-ray absorptiometry, the percentage of body fat increased from visit 2-3 (+0.9 +/- 1.6%; P = 0.02). Bone mineral content decreased from visit 2-3 (-0.02 +/- 0.04 kg; P = 0.02) and from visit 1-3 (-0.04 +/- 0.06 kg; P = 0.005). By computed tomography, the visceral adipose to sc adipose tissue ratio decreased from visit 1-3 (-0.02 +/- 0.05 ml; P = 0.02). We conclude that, compared with controls, women with breast cancer receiving modern adjuvant chemotherapy regimens show no significant changes in weight during the first year of their treatment. They do, however, appear to undergo unfavorable changes in body composition.