The purpose of this study was to evaluate the contributions of exercise, fitness, body composition, and calcium intake during adolescence to peak bone mineral density and bone structural measurements in young women. University Hospital and 75 healthy, white females in the longitudinal Penn State Young Women's Health Study were included. Body composition, total body, and hip bone mineral density (BMD) were measured by dual-energy X-ray absorptiometry (DXA), exercise scores by sports-exercise questionnaire during ages 12-18 years, and estimated aerobic capacity by bike ergometry. Section modulus values (a measurement of bending strength) cross-sectional area (CSA), subperiosteal width, and cortical thickness were calculated from DXA scan data for the femoral neck and femoral shaft. Calcium intakes were calculated from 39 days of prospective food records collected at 13 timepoints between ages 12 and 20 years; supplemental calcium intakes were included. Section moduli at the femoral neck and shaft were correlated significantly with lean body mass, sports-exercise scores (R(2) = 0.07-0.19, p < 0.05), and aerobic capacity (R(2) = 0.06-0.57, p < 0.05). Sports-exercise scores correlated with BMD at the femoral neck and shaft. Average total daily calcium intake at age 12-20 years ranged from 486 to 1958 mg/day and was not significantly associated with total or regional peak BMD or bone structure measures at 20 years of age. It was shown that achievable levels of exercise and fitness have a favorable effect on BMD and section modulus of the femoral neck and femoral shaft in young adult women, whereas daily calcium intake of >500 mg in female adolescents appears to have little, if any effect.