To evaluate the effects of genetic and environmental factors on the bone mass, we determined the bone mineral density (BMD) in the total body, the lumbar spine and in the femoral neck in 23 Japanese twin pairs including 21 monozygotic (MZ) pairs, applying dual-energy X-ray absorptiometry. In MZ pairs aged 20-49 years, highly significant intraclass correlation coefficients, ranging from 0.775 to 0.926, were observed at several sites, including the lumbar spine and the femoral neck, which suggests considerable contributions of genetic factors to the BMD. Neither intraclass correlation coefficients nor intrapair differences were found to change with increasing age in the present analysis. In female MZ pairs, exercises both at the present time and in the past were correlated with the BMD at several sites as well as body mass index in multiple regression analyses.