Some investigators consider body fat the main determinant of bone mass in women, and others, weight. Although genetics may be the primary determinant of peak bone mass, modifiable secondary factors such as nutrition and hormone exposure may significantly affect bone mass accretion during the second decade of life. To determine the principal determinants of bone mass, we analyzed total and regional bone mass (by dual-energy x-ray absorptiometry), weight, height, percentage body fat, and lean body mass in a group of 50 young women ranging in age from 14 to 18 years. Total-body bone mineral content, total-body bone mineral density, and total-body bone mineral density corrected for height, analyzed by multiple correlation, were not related to percentage body fat, although percentage body fat did correlate with the regional bone mineral content of the trunk (P < .001), legs (P < .01), and pelvis (P < .001). Lean body mass correlated with total-body bone mineral content and density, total-body bone mineral density corrected for height (P < .001), and regional bone mineral content of the trunk (P < .05), arms (P < .01), and legs (P < .001), but not with regional bone mineral content of the pelvis and head. Weight correlated significantly (P < .001) with all bone mass measurements except regional bone mineral content of the head, where the significance was P less than .05. Similar results were obtained by multiple regression analysis. These results indicate that weight, not body fat, is the main determinant of bone mass in postpubertal women.