Vitamin K is known to mediate carboxylation of glutamyl residues of osteocalcin. We evaluated the effects of vitamin K2 (menatetrenone) treatment (45 mg/day) for 48 weeks on the markers of bone formation and resorption, bone mineral density (BMD), and the incidence of vertebral fractures in 34 Japanese postmenopausal women (aged 48-82 years). Serum levels of alkaline phosphatase (ALP) increased gradually and became significant at 48 weeks after menatetrenone treatment, while urinary excretion of deoxypyridinoline (DPD) decreased transiently but significantly at 4 weeks. Serum levels of both intact osteocalcin (OC) and carboxylated OC (Gla-OC) increased rapidly and significantly within 4 weeks and sustained their high values up to 48 weeks after the treatment, while those of undercarboxylated OC (Glu-OC) decreased reciprocally. These results can be interpreted to suggest that Glu-OC was converted to Gla-OC in vivo. On the other hand, lumbar BMD values showed no significant change and only one subject with a previous vertebral fracture had one newly occurring vertebral fracture. These results indicate that menatetrenone treatment of postmenopausal women constantly elevates bone formation markers as well as converts Glu-OC to Gla-OC. Thus, vitamin K2 treatment may promote bone formation, at least as measured biochemically in these subjects.