Eighty-six healthy, young Caucasian 18-year-old men with no known disease or bone injury were recruited to this study at the United States Naval Academy. Change in bone mineral density (BMD) of the hip, lumbar spine, and distal tibia, and total body bone mineral content (TBMC) was measured by dual-energy X-ray absorptiometry (DXA). BMD and TBMC of these men were measured within 2 months (67 ± 3 days) of entering the Academy, and, at the end of their first, second, and fourth years. Hip BMD was unchanged during the study period (p > 0.05). Lumbar spine BMD increased 3% (p < 0.001) and distal tibia BMD increased 5% (p < 0.001). TBMC showed a 7.5% increase over the study period. In this group of young men, gain in BMD and TBMC is greatest to age 21 years, with minimal further increase after age 21. The significance of this rise in bone mass during early adulthood on risk for osteoporotic fractures in later life or its impact on exercise-related bone injuries is unknown and warrants further examination. Copyright (C) 2000 Elsevier Science Inc.