We explored factors that contributed to bone mineral density (BMD) in Japanese young women by quantifying the factors related to BMD. Between October 2003 and February 2004, we conducted a cross-sectional survey to study the status of nutritional intake and physical activity, and evaluated the various physical and serum parameters in relation to BMD. Subjects included 254 healthy female students who were 19-25 years old and were attending the Nursing School of Tokyo Women's Medical University, Japan. We measured the lumbar BMD (L2-L4) in these women. Multiple regression analysis was used to predict factors that contributed to current L2-L4 BMD. Our results showed that body mass index (BMI) (standardized regression coefficient = 0.45, P < 0.0001), past exercise habit (standardized regression coefficient = 0.15, P < 0.0059), and current total energy expenditure (standardized regression coefficient = 0.12, P < 0.03) were factors that significantly predicted the current L2-L4 BMD, with BMI as a key contributing factor. A BMI of 20.8 kg/m(2) allowed acquisition of young adult mean (YAM) irrespective of the total energy expenditure. In subjects with low BMI, L2-L4 BMD increased with higher current energy expenditure. A BMI of 20.8 kg/m(2) or greater and an energy expenditure of 32.9 METS-h/day or greater are required to acquire the YAM. We concluded that BMI and physical activity were factors that affected the BMD of Japanese young women.