Abstract. It is established in Japan that treatment with 1a-hydroxyvitamin D3 (alfacalcidol) slightly reduces bone turnover, sustains lumbar bone mineral density (BMD), and prevents osteoporotic vertebral fractures in postmenopausal women with osteoporosis, while vitamin K2 (menatetrenone)enhances ƒÁ-carboxylation of bone glutamic acid residues and secretion of osteocalcin, sustains lumbar BMD, and prevents osteoporotic fractures in patients with osteoporosis. Available evidence suggests that the effect of vitamin K2 on mineralization by human periosteal osteoblasts is enhanced in the presence of 1,25 dihydroxyvitamin D3 in vitro. The effect of vitamin K2 on BMD in ovariectomized rats is affected by the plasma 25-hydroxy vitamin D3 level in vivo, and is significant only when rats are fed a diet containing vitamin D3. Based on this line of evidence, combined treatment with alfacalcidol and menatetrenone for osteoporosis is surmised to be more effective than treatment with menatetrenonealone, and may have anabolic effects on osteoporotic bone. This combined treatment may increase bone formation as well as bone resorption over the mild anti-resorptive effect of alfacalcidol itself, and shows the greatest effect on lumbar BMD or the incidence of vertebral fractures in studies in which the mean age and years since menopause of subjects were low and the degree of osteoporosis was mild.It may be effective for mild postmenopausal osteoporosis in which age-related deterioration of trabecular bone properties remains below the threshold for vertebral fractures, even if bone resorption is increased and trabecular bone has deteriorated.