Objective: To assess the vitamin D status of healthy young people living in Northern Ireland and the effect of vitamin D supplementation on vitamin D status and bone turnover. Design: Double-blinded randomised controlled intervention study. Setting: University of Ulster, Coleraine, Northern Ireland. Subjects: In total, 30 apparently healthy students ( 15 male and 15 female subjects), aged 18 - 27 years, were recruited from the university, with 27 completing the intervention. Interventions: Subjects were randomly assigned, to receive either 15 mu g ( 600 IU) vitamin D-3 and 1500 mg calcium/day ( vitamin D group), or 1500 mg calcium/day ( control group) for 8 weeks between January and March. Vitamin D status, bone turnover markers, serum calcium and parathyroid hormone concentrations were measured at baseline and post intervention. Results: At baseline, vitamin D status was low in both the vitamin D group ( 47.9 ( s.d. 16.0)) and the control group ( 55.5 ( s.d. 18.6) nmol/l 25( OH) D). Post intervention vitamin D status was significantly higher in the vitamin D-treated group ( 86.5 ( s.d. 24.5)) compared to the control group ( 48.3 ( s.d. 16.8) nmol/l) ( P < 0.0001). There was no significant effect of supplementation on bone turnover markers or PTH concentrations. Conclusions: This study suggests that young adults in Northern Ireland do not consume an adequate daily dietary intake of vitamin D to maintain plasma vitamin D concentrations in the wintertime. A daily supplement of 15 mu g vitamin D-3 significantly increased vitamin D status in these individuals to levels of sufficiency. Achievement of an optimum vitamin D status among young adults may have future positive health implications.