It is hypothesized that vitamin D insufficiency in athletes might negatively affect sport performance. The objective of this study was to examine the effect of vitamin D ₃ supplementation on physical performance of adolescent swimmers with vitamin D insufficiency. Fifty-three adolescent competitive swimmers with vitamin D insufficiency (serum 25-hydroxyvitamin-D concentrations (25(OH)D)<30ng/ml, mean 24.2 ± 4.8ng/ml) were randomized to receive 2,000IU/day of vitamin D ₃ or placebo for 12 weeks. Swimming performance at several speeds, arm-grip strength, and one-legged balance, were measured before and after supplementation. The age-adjusted changes in performance variables during the study were compared between groups. 25(OH) D concentrations at study end were significantly higher in the vitamin group compared with the placebo group (29.6 ± 6.5ng/ml vs. 20.3 ± 4.2ng/ml, p < .001), yet only 48% of the vitamin group became vitamin D sufficient with this dosing. No between-group differences were found in the changes of the performance variables tested. No significant differences in performance were found between participants that became vitamin D sufficient, and those who did not. No significant correlation was found between the change in serum 25(OH)D and ageadjusted balance, strength or swimming performance at study end. Vitamin D ₃ supplementation that raised serum 25(OH)D concentrations by a mean of 9.3ng/ml above placebo in adolescent swimmers with vitamin D insufficiency, did not improve physical performance more than placebo.