Background: Vitamin D deficiency is common in the general public and athletic populations and may impair skeletal muscle function. We therefore assessed the effects of vitamin D supplementation on serum 25[OH]D concentrations and physical performance. Methods: 30 club-level athletes were block randomised (using baseline 25[OH]D concentrations) into one of three groups receiving either a placebo (PLB), 20 000 or 40 000 IU/week oral vitamin D for 12 weeks. Serum 25 [OH]D and muscle function (1-RM bench press and leg press and vertical jump height) were measured presupplementation, 6 and 12 weeks postsupplementation. Vitamin D deficiency was defined in accordance with the US Institute of Medicine guideline (0.05). Conclusions: Both 20 000 and 40 000 IU vitamin D supplementation over a 6-week period elevates serum 25 [OH]D concentrations above 50 nmol/l, but neither dose given for 12 weeks improved our chosen measures of physical performance.