The amount of skeletal mass acquired during adolescence is one of the most important determinants for the risk of postmenopausal and involutional osteoporosis. In both sexes, a large variance in bone mineral density (BMD) and content (BMC) is observed among healthy individuals at the beginning of the third decade. To determine the crucial pubertal years during which bone mass accumulation mainly occurs, we longitudinally monitored the gain in BMD/BMC at clinically important sites, such as lumbar spine and femoral neck, with respect to osteoporotic fracture risk. The changes in BMD (grams per cm2) and BMC (grams) were determined at 1-yr intervals at the level of lumbar spine vertebrae (L2-L4), femoral neck, and midfemoral shaft, using dual energy x-ray absorptiometry (Hologic QDR 1000), in 198 healthy adolescents (98 females and 100 males), aged 9-19 yr. Mean daily energy and calcium intakes, height, weight, and body mass index of the studied cohort were within the normal range for age. In females, the increment rate in BMD/BMC was particularly pronounced over a 3-yr period, i.e. from 11-14 yr of age. This increment dramatically fell after 16 yr and/or 2 yr after menarche. The mean gains in lumbar, femoral neck, and midfemoral shaft BMD were not statistically significant between 17-20 yr. In males, the gain in BMD/BMC was particularly high over a 4-yr period, i.e. from 13-17 yr. Then the increment rate markedly declined, but remained significant between 17-20 yr for L2-L4 BMD/BMC and midfemoral shaft BMD. In contrast, no significant increase was observed for femoral neck BMD. An impressive interindividual variation was observed between the yearly height increment and the bone mass accumulation. The bone mass-height gains relationship during puberty evolved according to a loop pattern, with maximal variance at Tanner stages P3-P4. This longitudinal study delineates the crucial pubertal years during which the skeletal mass accumulates at high, but various, rates at skeletal sites where the consequences of the osteoporosis are particularly dramatic. Furthermore, the results indicate that in a cohort of healthy females with apparently adequate intakes of energy and calcium, bone mass accumulation is drastically reduced by 16 yr of age in both lumbar spine and femoral neck.