BACKGROUND: vitamin D deficiency is a well-known cause of bone loss and fractures but its association, especially among the oldest old, with muscle weakness is less obvious. OBJECTIVE: to investigate the relationship between 25-hydroxyvitamin D (25-OHD) and muscle performance in persons aged 80 years and older. METHODS: baseline results of the Belfrail study, a prospective, population-based cohort study were used to study balance, grip strength and gait speed in relation to 25-OHD serum levels in 367 subjects. RESULTS: a sufficient 25-OHD serum level of 30 ng/ml or more was found in 12.8% of the population. The prevalence of vitamin deficiency (20-29 ng/ml), insufficiency (10-19 ng/ml) and severe insufficiency (<10 ng/ml) was 21.5, 33 and 32.7%, respectively. No significant relation between balance, gait speed and grip strength, and serum 25-OHD was detected neither in bivariate analysis nor after adjustment for age, gender, level of education, institutionalisation, smoking status, body mass index, co-morbidity, level of activity, season, CRP, renal function, serum calcium parathyroid hormone levels, vitamin D intake and use of loop or thiazide diuretics. CONCLUSION: in this cohort of octogenarians vitamin D deficiency was highly prevalent. We could not confirm the findings of previous studies showing an association between serum 25-OHD and physical performance in elderly.