BACKGROUND: vitamin D supplementation reduces the incidence of fractures in older adults. This may be partly mediated by effects of vitamin D on neuromuscular function. OBJECTIVE: to determine the effects of vitamin D supplementation on aspects of neuromuscular function known to be risk factors for falls and fractures. DESIGN: randomised, double-blind, placebo-controlled study. SETTING: falls clinic taking referrals from general practitioners and accident and emergency department. SUBJECTS: 139 ambulatory subjects (>/=65 years) with a history of falls and 25-hydroxyvitamin D (25OHD) </=12 microg/l. Intervention: patients were randomised to receive a single intramuscular injection of 600,000 i.u. ergocalciferol or placebo. OUTCOME MEASURES: assessments including biochemistry, postural sway, choice reaction time (CRT), aggregate functional performance time (AFPT), and quadriceps strength were carried out at baseline and 6 months post-intervention. RESULTS: baseline characteristics were comparable between both groups. 25OHD in the treatment group increased significantly at 6 months. AFPT deteriorated in the control group and improved in the intervention group, representing a significant difference between groups (+6.6 s versus -2.0 s, t = 2.80, P < 0.05). Similar changes were observed for CRT (-0.06 s versus +0.41 s, t = -2.52, P < 0.01) and postural sway (+0.0025 versus -0.0138, t = 2.35, P < 0.02). There was no significant difference in muscle strength change between groups (-10 N versus -2 N, t = -1.26, ns). A significant correlation between change in AFPT and change in 25OHD levels was observed (r = 0.19, P = 0.03). There was no significant difference in the number of falls (0.39 versus 0.24, t = 1.08, P = 0.28) or fallers (14 versus 11, P = 0.52) between two groups. CONCLUSIONS: vitamin D supplementation, in fallers with vitamin D insufficiency, has a significant beneficial effect on functional performance, reaction time and balance, but not muscle strength. This suggests that vitamin D supplementation improves neuromuscular or neuroprotective function, which may in part explain the mechanism whereby vitamin D reduces falls and fractures.