Interrelationships between percent of ideal body weight (%IBW), serum estrogen levels, and change in bone mineral density (delta BMD) and bone mineral content (delta BMC) were studied in 288 postmenopausal women aged 41-71 years who participated in a 2-year calcium supplement trial. The spine (L2-L4) and femoral neck were measured by dualphoton absorptiometry, and the radius was measured by single-photon absorptiometry. Years since menopause, calcium intake, and initial BMD or BMC were included as independent variables in two-phase regressions of delta BMD and delta BMD on %IBW. Increased %IBW protected against loss of spine BMD [regression slope estimate = 0.05, 95% C.I.: (0.03, 0.26)] and BMC in women up through about 106 %IBW but not in heavier women. Increased %IBW was not significantly related to delta BMD or delta BMC at the femoral neck or radius. Women above 106 %IBW had significant gains in spine and femoral neck area (P less than 0.05). Serum estrone and estradiol were positively correlated with delta BMD and delta BMC at the femoral neck only.