We have examined the relationships among calcium intake, mechanical stress (activity) and axial (lumbar) and peripheral (midradius) bone mass in a population of 60 young normal, healthy eumenorrheic women (age 25-34 years), selected on the basis of weight for height (within 20% of expected). In this population, vertebral bone mineral density was significantly related (r = 0.41, p less than 0.005) to activity pattern but not to calcium intake. Elimination of the effects of activity, however, disclosed a significant correlation between total calcium intake and vertebral bone mass (r = 0.36, p less than 0.02). Closer examination of this relationship suggested that vertebral bone mineral density did not appear to increase with calcium intakes above 800-1000 mg/day, implying a threshold effect and vertebral density was better expressed as a function of the logarithm of calcium intake (r = p less than 0.01). Radial mineral content was not related to activity (which was mostly walking and running), but it was significantly affected by calcium intake. These data suggest that skeletal status in the vertebrae in young women may be influenced by modulation of mechanical stress. However, bone mineral density is probably also dependent on nutritional status such that the optimum calcium intake for this effect is 800-1000 mg/day. If such effects, observed here cross-sectionally, can be substantiated in longitudinal studies and persist into the aging postmenopausal population, then the average age of onset of osteoporotic fractures would be expected to be delayed approximately 10 years, by relatively simple modifications of diet and activity.