Skeletal integrity is related to the balance between bone formation and bone resorption, which is mediated by factors including genetics, hormones, loading, and calcium supply. The extent to which these determinants of bone environment influence markers of bone turnover remains unclear. PURPOSE This investigation was conducted to evaluate the importance of 17β-HSD1 genotype, estradiol (E2), androstenedione (AD), testosterone (T), insulin-like growth factor-I (IGF-I), physical activity, and calcium intake to concentrations of markers of bone formation (osteocalcin, OC) and resorption (cross-laps, LAPS) in college-aged young women. METHODS Fasting serum was obtained from 32 women aged 21.6 ± 3.4 years (X ±SD) for the determination of OC, E2, AD, T, and IGF-I on day 11 ± of an ovulatory menstrual cycle. OC and IGF-I concentrations were determined by immunoradiometric assay, while E2, AD, and T were determined by radio-immuno assay. 17β-HSD1 genotype was determined by PCR as A/A, A/G, or G/G. LAPS were determined from urine by enzyme-linked immunosorbent assay. Physical activity and dietary calcium intake were derived from questionnaires. Stepwise linear regression analysis determined the proportion of the variance in OC and LAPS accounted for by the variables. Significance was set at p < 0.05. RESULTS For osteocalcin, E2 accounted for 18.8% of the variance (p = 0.012), with physical activity accounting for an additional 16.3% (p = 0.011). The variance in LAPS was explained by IGF-I concentrations (11.4%, p = 0.044) and calcium intake (11.1%, p = 0.039). None of the other variables were significant predictors of either marker. CONCLUSION It is evident from the results that diverse factors are important in determining rates of bone formation and resorption in young women. These results underscore the complex roles of estrogen, physical activity, IGF-I and calcium in the maintenance of skeletal integrity. Supported by USC James H. Zumberge Faculty Research and Innovation Fund, USC MFWA Research Fund, and BCRP 1KB-0060