Abnormalities in plasma vitamin D metabolites and an increased prevalence of osteomalacia have been described in elderly patients sustaining a fracture of the femoral neck. In order to investigate whether the plasma concentrations of the vitamin D metabolites are normal, and whether vitamin D deficient osteomalacia in patients with femoral fracture can be diagnosed using biochemical criteria alone, we have studied before and after 7 days of 40 micrograms oral 25-hydroxyvitamin D3 elderly patients admitted to hospital with a femoral fracture, elderly patients undergoing elective replacement of the femoral head and elderly control patients in hospital with no clinical evidence of bone disease. Plasma 25-hydroxyvitamin D (25(OH)D) and 24,25-dihydroxyvitamin D increased after 7 days of oral 25-hydroxyvitamin D3 to the same levels in the three groups, but in contrast to the controls there was no significant increase in plasma 1,25-dihydroxyvitamin D or radiocalcium absorption in femoral fracture and hip replacement patients. However, when femoral fracture patients were restudied 6-12 months after fracture, plasma 1,25-dihydroxyvitamin D increased after oral 25-hydroxyvitamin D3 to the same extent as it had in the control patients. We conclude that reduced calcium absorption due to low plasma 25(OH)D levels, i.e., vitamin D insufficiency is common in all elderly patients. Furthermore biochemical criteria for diagnosis of vitamin D-deficient osteomalacia are of very limited use at the time of fracture in elderly patients since there is a failure of production of 1,25 dihydroxyvitamin D which resolves within 6-12 months of the fracture. This failure makes the 1,25-dihydroxyvitamin D response to oral 25(OH)D an unreliable guide to the presence of vitamin D-deficient osteomalacia at the time of fracture. The abnormality in 1,25-dihydroxyvitamin D is also present in patients undergoing hip replacement surgery, and is therefore unlikely to be involved in the aetiology of femoral neck fracture. It may, however, contribute to the morbidity after fracture.