It is common for women undergoing treatment for breast cancer to gain weight, although the characteristics of the weight change have not been described. We investigated the changes in abdominal fat accumulation that accompanied the change in weight associated with treatment for breast cancer in longitudinal and cross-sectional clinical studies in 34 women aged 39-73 y with early-stage primary breast cancer. Computerized tomography scans of abdominal subcutaneous and visceral adipose depots, bioelectrical impedance measurements of body fat mass, and measurements of body weight and girth were obtained early in the course of treatment and 6 mo later (longitudinal study; n = 8) or within 12 mo of treatment (cross-sectional study; n = 26). The longitudinal study found that, irrespective of the direction of weight change, seven of eight women gained body fat and lost lean body mass. In the five women who gained weight (median: 3.2 kg) two lost and three gained subcutaneous adipose fat (median: 19%) whereas all gained visceral fat (median: 23%). In the cross-sectional study 19 women gained weight and 7 lost weight or had stable weight since diagnosis. Change in weight was correlated with abdominal subcutaneous adipose fat (r = 0.39; P = 0.06) and hip circumference (r = 0.43; P = 0.03) but not abdominal visceral fat, the ratio of subcutaneous to visceral fat, or the ratio of waist to hip size. In the longitudinal sample, weight gain resulted in a variable response in subcutaneous adipose volumes but a consistent increase in visceral adipose depot. Although these results are preliminary, it appeared that regardless of weight gain or loss women were likely to lose lean body mass and gain fat mass during treatment for breast cancer.