Because of its role in osteoblastic metabolism, vitamin K has been studied with respect to bone. However, there has been limited research examining the influence of long-term vitamin K supplementation on bone mineral density (BMD). Therefore, the purpose of this study was to assess the impact of 6 months of vitamin K supplementation on BMD and biomarkers of bone in pre- and perimenopausal women. Based on previous work, we hypothesized that vitamin K would improve BMD and biochemical markers of bone formation. A double-blind, placebo-controlled, randomized trial is an effective way to study the impact of long-term supplementation. Thus, 14 pre- and perimenopausal women, 25 to 50 years of age, were randomly assigned to an experimental group (E) that received 600 microg/d of vitamin K in the form of phylloquinone (K(1)) or a control group (C) that received identical-looking placebo tablets. Regional BMD and percent body fat, measured by dual-energy x-ray absorptiometry, and serum osteocalcin and urinary N-telopeptide levels were all assessed at 0, 3, and 6 months. When BMD was measured across time, C had a significant increase (P = .011) in greater trochanter BMD compared to E. The E group had a nonsignificant increase (P = .067) in shaft BMD compared to the C group. There was no significant difference between E and C in serum osteocalcin concentrations over time. Urinary N-telopeptide levels increased significantly over time in E compared to C (P = .008). Six months of 600 microg/d vitamin K(1) supplementation did not improve regional BMD in this group of pre- and perimenopausal women.