In this cross-sectional study, the bone mineral density of the lumbar spine, the back extensor strength, and the level of physical activity were determined in 68 healthy, postmenopausal women. Physical activity was determined by a graded questionnaire and varied from sedentary to heavy vocational and avocational activity levels. Bone mineral density (L2 through L4) was measured with dual-photon absorptiometry. Back extensor strength was determined with use of a strain-gauge dynamometer. Bone mineral density ranged from 0.77 to 1.49g/cm2 (mean +/- SD, 1.06 +/- 0.18g/cm2). Back extensor strength ranged from 37 to 145 lb (84.6 +/- 23.5 lb). Physical activity scores ranged from 3 to 13 (8.0 +/- 2.7). Statistical analysis demonstrated a significant positive correlation (r = 0.34, p less than 0.005) between bone mineral density and back extensor strength. Bone mineral density was also positively correlated with the level of physical activity (r = 0.24, p less than 0.05). There was a significant positive correlation between back extensor strength and level of physical activity (r = 0.37, p less than 0.005). The data suggest that levels of physical activity and back muscle strength may be contributors to the bone mineral density of vertebral bodies.