Peak bone mass at skeletal maturity may be an important factor in the relative quantity of skeletal mass in old age. We have studied bone mineral in 46 young men, 28 of whom engage in regular and vigorous exercise programs. Spinal trabecular bone mineral density and spinal integral bone mineral content are significantly greater in the exercise group as compared with the 18 control subjects. Of the exercise group, subjects participating in both aerobic and weight-bearing regimens have the greatest spinal bone mineral mass, followed by those engaging in strictly weight-bearing exercise and those in a primarily aerobic program. An analysis of variance across all subject groups, including the control group, shows a significant difference in spinal trabecular bone density based on the type of physical activity.