The role of vitamin D in the prevention of hip fracture is controversial and protective effects appear to be confined to the institutionalized and elderly population. The aim of this study was to assess the interaction of age and body mass index (BMI) on the effects of vitamin D on the risk of hip fracture in the community. We studied 1634 women with low-energy fractures of the hip aged 50 years or older, and 3532 age-matched controls from 14 centres in six Mediterranean countries (the MEDOS study), with a structured retrospective questionnaire. The use of vitamin D supplements was associated with a modest and non-significant decrease in the risk of hip fracture (RR = 0.74; 95% confidence interval (CI) 0.53-1.03; p = 0.07). The risk reduction was influenced by age and body mass. Women aged above 80 years had a significant decrease in the relative risk of hip fracture (RR = 0.63; 95% CI 0.40-0.98) as did women with a BMI below 20 kg/m2 (RR = 0.45; 95% CI 0.24-0.84). Elderly women were more likely to have a low BMI but the elderly did not appear to benefit from vitamin D where their BMI was 20 kg/m2 or higher. The findings could not be explained by differences in sun exposure or in physical exercise. We conclude that the use of vitamin D for the prevention of hip fracture might usefully be targeted to the frail and elderly.