Objective- Although vitamin D and bone metabolism are closely related, few studies have addressed the effects of vitamin D status on bone in men at time of peak bone mass. The objectives of this study were to evaluate the prevalence of vitamin D inadequacy in a cross-sectional study in young men and the effects of vitamin D and parathyroid hormone (PTH) on bone mass, bone markers and metabolic function. Design and Participants- The study population consisted of 783 men aged 20-29 years. Measurements- Bone mineral density (BMD) of the total hip, femoral neck and lumbar spine was measured. dual-energy X-ray absorptiometry was used to evaluate total body fat mass (BFAT). Visceral fat mass and abdominal subcutaneous fat mass (ViFM and ScFM) were assessed using magnetic resonance imaging. A radioimmunoassay was used to measure the level of 25-hydroxy vitamin D (25OHD). Results- The prevalence of vitamin deficiency (serum 25OHD < 50 nm) was 6•3% during summer and 43•6% during winter. Serum 25OHD was associated with BMD at all sites and inversely associated with bone-specific alkaline phosphatase and directly with carboxyterminal telopeptide of type-1-collagen. 25OHD and PTH were inversely associated with BFAT, whereas 25OHD also was inversely associated with body mass index, waist-hip ratio, ViFM and ScFM after adjustment for confounders. The associations were found only to be present in participants with insufficient levels of 25OHD. 25-Hydroxy vitamin D and PTH were inversely related to insulin resistance in vitamin-insufficient participants only. No associations between PTH or 25OHD and blood pressure were noted. Conclusion- The study showed a high prevalence of 25OHD deficiency in young, northern European men, which was significantly associated with decreased BMD. PTH and 25OHD were found to be inversely related to the markers of insulin resistance.