Oral administration of vitamin K was reported to increase bone mineral density. However, the possible role of vitamin K in the pathogenesis of osteoporosis still remains unclear. Therefore, we measured the serum concentration of vitamin K1 and K2 (menaquinone-4, 7, 8) in 24 elderly women with osteoporotic vertebral compression fracture and in 36 elderly women without fracture. Major forms of vitamin K present in sera in this study were vitamin K1 and menaquinone-7. On the other hand, serum menaquinone-4 and -8 were undetectable in most women. Serum concentration of menaquinone-7 was significantly lower in women with fracture than in those without fracture (3.29 +/- 3.63 ng/ml vs 6.26 +/- 5.62, mean +/- SD, respectively), while no difference was found in serum vitamin K1 concentration (0.837 +/- 0.620 ng/ml vs 0.820 +/- 0.686, respectively). There was no difference between both groups in background data such as age, body height, body weight, and body mass index, as well as serum level of calcium, inorganic phosphate, creatinine, albumin, and alkaline phosphatase. These results suggest the possibility that deficiency of vitamin K, particularly that of menaquinone-7, is one of the risk factors for developing osteoporosis.