Circulating estradiol (E2), estrone (E1), adrostenedione, and testosterone levels were measured in 40 normal postmenopausal women of widely varying body weights. The fasting urinary calcium to creatinine ratio (Ca:Cr) was also quantitated as an index of bone resorption. Significant positive correlations of E2 and E1 were found with body weight and correlations of E2 and E1 were found with body weight and percent ideal weight but not with height, age, or years since menopause. No correlations were observed between circulating androstenedione and testosterone levels and any of these characteristics. Significant negative correlations were noted between Ca:Cr and percent ideal weight and between Ca:Cr and E2 and E1 concentrations. Administration of 10 micrograms ethinyl E2 to 10 postmenopausal subjects for 30 days reduced Ca:Cr to the level observed in 20 premenopausal women. These data suggest that body weight can influence urinary calcium excretion. It is possible that the reduced amounts of endogenous estrogen found in conjunction with low body weight may be a factor contributing to the greater loss of urinary calcium and the more frequent occurrence of osteoporosis in slender postmenopausal women.