Objective: To identify a relation between loss of muscle strength and vitamin D deficiency in ambulatory elderly persons not receiving vitamin D supplementation. Design: Cross-sectional study. Setting: All measurements were taken at the Department of Geriatrics, University Hospital, Basel, Switzerland. Subjects: Three hundred nineteen patients (103 women, 216 men) selected by random sampling from participants in an ongoing interdisciplinary study on aging (mean age for women, 74.2yrs; for men, 76.7yrs). Outcome Measures: Leg extension power (LEP) and body mass index (BMI); serum values of 25-hydroxyvitamin D [calcidiol, 25(OH)D], 1,25-hydroxyvitamin D [calcitriol, 1,25(OH)2D], and intact parathyroid hormone (iPHT). Results: Twelve percent of women and 18% of men had 25(OH)D values below the normal range (<12ng/mL). Muscle strength was lower in older subjects (female: r = - .35; p = .0005/male: r = -.48; p <.0001) and was lower in women than in men (p <.0001). In men both 25(OH)D and 1,25(OH)2D was significantly correlated with LEP (r = 0.24; p = .0004/r =. 14; p = .045). In women, only 1,25(OH)2D was significantly correlated with LEP (r = 0.22; p = .034). In an ANCOVA including all participants and explaining LEP by sex, age, BMI, 1,25(OH)2D, 25(OH)D, and iPTH, all factors showed significant effects except 25(OH)D and iPTH (r2 = .41). Conclusion: Muscle strength declined with age in ambulatory elderly people and showed modest, but significant, positive correlation with 1,25(OH)2 vitamin D in both sexes and with 25(OH)D in male subjects. Therefore vitamin D deficiency appears to contribute to the age-related loss of muscle strength, which might be more pronounced in institutionalized elderly people with a high prevalence of vitamin D deficiency.