The relationship between physical activity and bone mass was investigated in 24 healthy, white, premenopausal women (mean age [+/- SE], 39.0 +/- 1.39 years). Physical activity was determined by a sensor that measures movement of the trunk, and bone mineral levels were determined by means of single- and dual-photon absorptiometry and neutron activation analysis. Total physical activity levels were related both to bone mineral density of the spine (r = .41) and to total body calcium levels (r = .51). There was no significant relationship between the bone density of the distal portion of the radius and activity (r = .20). Nonparametric analysis and stepwise multiple regression analysis revealed negative correlations between cigarette smoking and bone density of the spine and radius. These data suggest that the level of physical activity in sedentary white women may be a determinant of peak total skeletal mass and bone density of the spine.