The risk of vitamin D insufficiency is increased in persons having limited sunlight exposure and dietary vitamin D. Supplementation compliance might be improved with larger doses taken less often, but this may increase the potential for side effects. The objective of the present study was to determine whether a weekly or weekly/monthly regimen of vitamin D supplementation is as effective as daily supplementation without increasing the risk of side effects. Participants were forty-eight healthy adults who were randomly assigned for 3 months to placebo or one of three supplementation regimens: 50Â μg/d (2000Â IU/d, analysed dose 70Â μg/d), 250Â μg/week (10Â 000 IU/week, analysed dose 331Â μg/week) or 1250Â μg/week (50Â 000Â IU/week, analysed dose 1544Â μg/week) for 4 weeks and then 1250Â μg/month for 2 months. Daily and weekly doses were equally effective at increasing serum 25-hydroxyvitamin D, which was significantly greater than baseline in all the supplemented groups after 30Â d of treatment. Subjects in the 1250Â μg treatment group, who had a BMI >26Â kg/m2, had a steady increase in urinary Ca in the first 3 weeks of supplementation, and, overall, the relative risk of hypercalciuria was higher in the 1250Â μg group than in the placebo group (P=Â 0·01). Although vitamin D supplementation remains a controversial issue, these data document that supplementing with ≤Â 250Â μg/week (Â ≤Â 10Â 000Â IU/week) can improve or maintain vitamin D status in healthy populations without the risk of hypercalciuria, but 24Â h urinary Ca excretion should be evaluated in healthy persons receiving vitamin D3 supplementation in weekly single doses of 1250Â μg (50Â 000Â IU).