Weight gain is a common problem amongst women receiving adjuvant chemotherapy for early breast cancer. We undertook a study to determine the causes of this weight gain. Prospective measurements of body mass and composition (skinfolds, bioelectrical impedance, total body potassium), energy balance (resting energy expenditure dietary intake, and physical activity), were determined in 17 women during and in the 6 months after commencing adjuvant chemotherapy. Women gained significant amounts of weight (5.0 +/- 3.8; p < 0.01) and body fat (7.1 kg +/- 4.5; p < 0.01) over the year. Waist circumference (5.1 +/- 4.5 cm; p < 0.01) and abdominal skinfold (16.2 +/- 10 mm; p < 0.01) were also increased but there was a decline in fat free mass (FFM); 1.7 +/- 2.5 kg. Women due to receive adjuvant chemotherapy had a greater resting energy expenditure (REE) compared with healthy subjects (n = 21); 100.5 +/- 8.0% Harris Benedict compared to 94.5 +/- 8.4% Harris Benedict (p = 0.05). REE declined by 3% during adjuvant chemotherapy (p < 0.05), and remained depressed until at least 3 months posttreatment. There were no significant changes in dietary intake or physical activity over the year. Failure of women to reduce their energy intake to compensate for the decreased energy requirement may account for some of the weight gain. Treatment of adjuvant chemotherapy causes gain of body fat because of reduced energy expenditure, and the failure of women to reduce their energy intake to compensate for the decline in energy requirement during and in the 6 months posttreatment. Since weight gain impacts on survival, patients should be counselled to reduce energy intake and exercise during and after adjuvant treatment.