The effect of the combined administration of vitamin D3 and vitamin K2 on bone mineral density (BMD) of the lumbar spine was examined in postmenopausal women with osteoporosis. Ninety-two osteoporotic women who were more than 5 years after menopause, aged 55-81 years, were randomly divided into four administration groups: vitamin D3 (1alpha hydroxyvitamin D3, 0.75 microg/day) (D group; n = 29), vitamin K2 (menatetrenone, 45 mg/day) (K group; n = 22), vitamin D3 plus vitamin K2 (DK group, n = 21), and calcium (calcium lactate, 2 g/day) (C group; n = 20). BMD of the lumbar spine (L2-L4) was measured by dual energy X-ray absorptiometry at 0, 1, and 2 years after the treatment started. There were no significant differences in age, body mass index, years since menopause, and initial BMD among the four groups. One-way analysis of variance (ANOVA) with repeated measurements showed a significant decrease in BMD in the C group (P < 0.001). Two-way ANOVA with repeated measurements showed a significant increase in BMD in the D and K groups compared with that in the C group (P < 0.05 and P < 0.001, respectively), and a significant increase in BMD in the DK group compared with that in the C, D, and K groups (P < 0.0001, P < 0.05 and P < 0.01, respectively). These findings indicate that combined administration of vitamin D3 and vitamin K2, compared with calcium administration, appears to be useful in increasing the BMD of the lumbar spine in postmenopausal women with osteoporosis.