Osteoporosis causes premature disability among millions of elderly people in the United States, particularly women. Exercise helps to maintain bone mass, yet it remains unclear what type of physical activity during what age periods is most beneficial for bone mass and density. This cross-sectional study investigated the relationship between different measures of lifetime physical activity and bone mineral density (BMD) in 25 premenopausal women (mean age 41 years, range 28-50 years). BMD of the total, entire axial, and entire peripheral skeleton were measured by dual energy x-ray absorptiometry (DXA). Lifetime history of physical activity was obtained by a structured interview, and estimates of lifetime weight-bearing exercise, total exercise, total weight-bearing physical activity (including occupational and household activities), and perceived physical activity were computed. Adjustments were made for current body weight and height and lifetime milk consumption. Lifetime weight-bearing exercise was significantly correlated with total and peripheral BMD (total BMD: r = 0.54; peripheral BMD: r = 0.54, all p < 0.05). Total weight-bearing physical activity, including occupational and household activity, was associated with total BMD (r = 0.51, p < 0.05). Physical activity during early age periods was more strongly associated with BMD at all sites than was physical activity in the past 2 years. Our findings suggest that lifetime weight-bearing exercise is more strongly related to BMD of the total and peripheral skeleton than is nonweight-bearing exercise. The inclusion of nonweight-bearing activity resulted in a weakening of the association. Weight-bearing household and occupational activity appeared to be related to BMD. Our results support recommendations to increase physical activity throughout life, particularly during early ages, as a means of osteoporosis prevention.