BACKGROUND AND AIMS: Insufficient vitamin D status, commonly found in older people, has been associated with muscle weakness which, in old age, impairs mobility and is a risk factor for falling. In a randomized, double-blind placebo-controlled trial, we tested the hypothesis that vitamin D + calcium supplementation improves muscle strength and mobility, compared with calcium mono-therapy in vitamin D-insufficient female geriatric patients. METHODS: Seventy female geriatric patients >65 years of age with serum 25-hydroxyvitamin D3 (25OHD) concentrations between 20 and 50 nmol/L, visiting an outpatient geriatric department, were included. Participants received either cholecalciferol 400 IU/day + calcium 500 mg/day (D/Cal group) or a placebo + calcium 500 mg/day (Plac/Cal group) for 6 months. At baseline and 6 months, muscle strength, power and functional mobility were tested. RESULTS: At baseline, 25OHD was significantly (p<0.05) associated with knee extension strength (r=0.42), handgrip strength (r=0.28), leg extension power (r=0.34), Timed Get Up and Go (r=-0.31) and Modified Cooper test (r=0.44). At 6 months, a significant difference in 25OHD (77.2 vs 41.6 nmol/L, p<0.001) and 1,25OHD was found between the two groups. Significantly improving vitamin D status in the D/Cal group compared with the Plac/Cal group did not result in a significant difference in strength or functional mobility between the two groups. CONCLUSIONS: Daily 400 IU vitamin D + 500 mg calcium supplementation is not enough to significantly improve strength or mobility in vitamin D-insufficient female geriatric patients.