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Context:Vitamin D insufficiency is common among the adolescent population and may have implications for health outcomes in later life. Few studies have investigated the role of vitamin D status in muscle function and cardiorespiratory fitness (CRF) during adolescence.Objective:The aim of this study was to investigate the association between vitamin D status and fat-free mass (FFM), muscle strength, muscle power, and CRF in a representative sample of adolescents from Northern Ireland.Design, Setting, and Participants:This was a cross-sectional study involving 1015 adolescents (age 12 and 15 y), who had 25-hydroxyvitamin D [25(OH)D] data available as part of the Young Hearts Study 2000.Main Outcome Measures:The main outcome measures of this study were FFM (kg) and FFM corrected for height [fat-free mass index; FFM (kg)/height (m2)], muscle strength (kg), peak muscle power (kW) and VO2max (CRF; mL/kg/min).Results:Multinomial regression analyses, controlling for environmental and lifestyle factors, demonstrated that boys age 15 years in the highest tertile of standardized serum 25(OH)D concentration (> 51 nmol/L) had significantly higher muscle strength (β = 3.90; P≤ .001) compared with those in the lowest tertile (< 32 nmol/L). These results were not evident in any other age-sex group and vitamin D status was not significantly associated with muscle power or CRF in any of the four age-sex groups.Conclusion:These results support a role for vitamin D in muscle function in adolescent males and suggest the need for more research in this vulnerable age group.