Reports from cross-sectional comparisons, nonrandomized prospective studies, and relatively small clinical trials indicate that postmenopausal hormone therapy may slightly decrease the amount of weight typically gained by women during the decade following menopause. Despite this, widespread belief remains that hormone therapy may cause weight gain. We use data from the Postmenopausal Estrogen/Progestin Interventions trial to characterize the impact of postmenopausal hormone therapy on weight and fat distribution and to examine the consistency of this impact among subgroups of women defined by lifestyle, clinical, and demographic factors. The Postmenopausal Estrogen/Progestin Interventions trial was a 3-yr, placebo-controlled, randomized clinical trial of 875 women assessing the effects on cardiovascular risk factors of four hormone regimens: oral conjugated equine estrogen (CEE) therapy (0.625 mg daily alone), CEE in combination with medroxyprogesterone acetate (2.5 mg daily), CEE in combination with medroxyprogesterone acetate (10 mg daily on days 1-12), and CEE in combination with micronized progesterone (200 mg daily on days 1-12). Women randomly assigned to CEE with or without a progestational agent averaged 1.0 kg less weight gain at the end of 3 yr (P = 0.006) than those assigned to placebo. Assignment to CEE was also associated with averages of 1.2 cm less increase in waist girth (P = 0.01) and 0.3 cm less increase in hip (P = 0.07) girth. In regression models that included weight change as a covariate, none of these differences reached statistical significance. There were no significant differences in weight or girth changes among any of the four active hormone regimens. After accounting for the effects of assignment to active hormone therapy and baseline weight, older age (P 0.008) and higher physical activity level at baseline (P = 0.002) were also independently predictive of less weight gain. The impact of hormone therapy on weight gain was similar among subgroups, except for those defined by baseline smoking status (P = 0.04) and physical activity level at home (P = 0.02). Factors that were independently associated with smaller increases in girths were: for waist, greater overall activity (P = 0.005) and Hispanic ethnicity (P = 0.02); and for hip, work activity (P = 0.003) and greater alcohol consumption (P = 0.03). None of these factors significantly affected the observed overall relationships between estrogen and changes in girth.