Many types of vitamin supplements are available on the market, but little is known about whether cholecalciferol obtained from fat-containing capsules differs in bioavailability from that of solid tablets. Our objective was to test whether 4 weeks of daily supplementation with 10 mug cholecalciferol given as a fish oil capsule produces a larger increase in serum 25-hydroxyvitamin D (s-25(OH)D) concentration compared with the same dose of cholecalciferol given as a multivitamin tablet. A total of seventy-four healthy subjects aged 19-49 years were initially included and fifty-five of these completed the study and fulfilled the inclusion criteria. After completing a self-administered questionnaire about diet and sunshine exposure and having a non-fasting venous blood sample drawn, participants were randomised to receive daily multivitamin tablets (n 28) or fish oil capsules (n 27), each containing equal doses of cholecalciferol. A second blood sample was drawn after 28 d. Mean baseline s-25(OH)D was 40.3 (sd 22.0) nmol/l in the multivitamin group and 48.5 (24.8) nmol/l in the fish oil group. When controlling for baseline s-25(OH)D, mean 4-week increase in s-25(OH)D was 35.8 (95 % CI 30.9, 40.8) nmol/l in the multivitamin group and 32.3 (95 % CI 27.3, 37.4) nmol/l in the fish oil group; the mean difference was 3.5 (95 % CI - 3.6, 10.6) nmol/l (P = 0.33). The results were unaltered by statistical adjustment for BMI, ethnic background, age and sex. We conclude that fish oil capsules and multivitamin tablets containing 10 microg cholecalciferol administered over a 4-week period produced a similar mean increase in s-25(OH)D concentration.