Twenty elderly osteoporotic women with vertebral fracture(s) were randomly allocated to two groups; women in the MK(4) group received calcium with menaquinone 4 (MK(4)) at a dose of 45 mg/day for 2 weeks, and women in the control group received calcium alone for the same period. Serum intact osteocalcin (OC) and undercarboxylated osteocalcin (uc-OC) levels were measured by immunoradiometric assay and enzyme immunoassay, respectively, at baseline and on the 7th and 14th days following the start of the treatment. There were no differences in the baseline data including age, weight, phylloquinone, menaquinone 4, menaquinone 7, OC, and uc-OC levels between the MK(4) group and the control group. Administration of MK(4) significantly raised the MK(4) level from 0.20 +/- 0.10 (mean +/- SE) pg/ml to 15.09 +/- 5.62 pg/ml ( P < 0.04), and reduced serum uc-OC levels from 2.80 +/- 0.93 ng/ml to 1.76 +/- 0.56 ng/ml ( P < 0.05) at the end of the study, respectively. No significant changes in these levels were observed in the control group. Serum OC levels were stable during the period in both groups. In this randomized prospective study, the MK(4) group shows a reduction in the serum uc-OC level within 2 weeks without any significant change in OC, suggesting that the uc-OC is changed to carboxylated OC. This early effect of MK(4) on bone metabolism may be estimated by the measurement of serum uc-OC in elderly osteoporotic women with vertebral fractures.