Purpose  The effect of 40 μg (1,600 IU) per day of vitamin D₃ on serum 25-hydroxyvitamin D (25(OH)D) and markers of bone and mineral metabolism was evaluated. Methods  This intervention study was designed as a double-blind randomised controlled trial. Forty-five community-dwelling subjects (32 females), age 55–84 years, at 58° North latitude were supplemented for 1 year with 40 μg vitamin D₃ plus 1,000 mg calcium per day, or with 1,000 mg calcium per day for controls. Safety parameters and 25(OH)D, intact parathyroid hormone (PTH), ionized calcium, bone-specific alkaline phosphatase (BALP), and tartrate-resistant acid phosphatase isoform 5b (TRACP5b) were measured over the study period. Results  All subjects supplemented with vitamin D₃ reached a 25(OH)D level above 50 nmol/L. Mean (SD) serum 25(OH)D increased from 50.4 (13.5) nmol/L to 84.2 (17.5) nmol/L, range 55.0–125.0 nmol/L in the vitamin D₃ supplemented group and the corresponding levels for the control group were 47.3 (14.1) nmol/L and 45.7 (13.4) nmol/L, range 26.0–73.0 nmol/L. No serious adverse event was recorded and the highest 25(OH)D level reached, 125.0 nmol/L, is well below toxic levels. BALP and TRACP5b did not change significantly over the study period. Conclusions  This trial suggests that a daily supplementation with 40 μg vitamin D₃ is sufficient to secure a 25(OH)D level of 50 nmol/L. No side effects were observed in the study group.