CONTEXT: There has been a resurgence of vitamin D deficiency among infants, toddlers, and adolescents in the United Kingdom. Myopathy is an important clinical symptom of vitamin D deficiency, yet it has not been widely studied. OBJECTIVE: Our objective was to investigate the relationship of baseline serum 25 hydroxyvitamin D [25(OH)D] concentration and PTH with muscle power and force. DESIGN: This was a cross-sectional study. SETTING: The study was community based in a secondary school. PARTICIPANTS: A total of 99 post-menarchal 12- to 14-yr-old females was included in the study. MAIN OUTCOME MEASURES: Jumping mechanography to measure muscle power, velocity, jump height, and Esslinger Fitness Index from a two-legged counter movement jump and force from multiple one-legged hops was performed. Body height, weight, and serum concentrations of 25(OH)D, PTH, and calcium were measured. RESULTS: Median serum 25(OH)D concentration was 21.3 nmol/liter (range 2.5-88.5) and PTH 3.7 pmol/liter (range 0.47-26.2). After correction for weight using a quadratic function, there was a positive relationship between 25(OH)D and jump velocity (P = 0.002), jump height (P = 0.005), power (P = 0.003), Esslinger Fitness Index (P = 0.003), and force (P = 0.05). There was a negative effect of PTH upon jump velocity (P = 0.04). CONCLUSION: From these data we conclude that vitamin D was significantly associated with muscle power and force in adolescent girls.