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Background/Objectives:: To assess the prevalence of vitamin D deficiency in Dutch athletes and to define the required dosage of vitamin D3supplementation to prevent vitamin D deficiency over the course of a year. Subjects/Methods:: Blood samples were collected from 128 highly trained athletes to assess total 25(OH)D concentration. Of these 128 athletes, 54 male and 48 female athletes (18–32 years) were included in a randomized, double blind, dose–response study. Athletes with either a deficient (<50?nmol/l) or an insufficient (50–75?nmol/l) 25(OH)D concentration were randomly assigned to take 400, 1100 or 2200 IU vitamin D3 per day orally for 1 year. Athletes who had a total 25(OH)D concentration above 75?nmol/l at baseline continued with the study protocol without receiving vitamin D supplements. Serum total 25(OH)D concentration was assessed every 3 months, as well as dietary vitamin D intake and sunlight exposure. Results:: Nearly 70% of all athletes showed an insufficient (50–75?nmol/l) or a deficient (<50?nmol/l) 25(OH)D concentration at baseline. After 12 months, serum 25(OH)D concentration had increased more in the 2200 IU/day group (+50±27?nmol/l) than the sufficient group receiving no supplements (+4±17?nmol/l; P<0.01) and the 1100 IU/day group (+25±23?nmol/l; P<0.05). Supplementation with 2200 IU/day vitamin D resulted in a sufficient 25(OH)D concentration in 80% of the athletes after 12 months. Conclusions:: Vitamin D deficiency is highly prevalent in athletes. Athletes with a deficient or an insufficient 25(OH)D concentration can achieve a sufficient 25(OH)D concentration within 3 months by taking 2200 IU/day.