Vitamin D deficiency is frequently observed in patients with Crohn's disease and may be associated with an increased risk of development of metabolic bone disease. To estimate the incidence of metabolic bone disease by noninvasive methods, 31 patients (17-75 years old) with Crohn's disease and low 25-hydroxyvitamin D (25-OHD) levels in winter were investigated in the following summer by measuring the bone mineral content (BMC) of the distal radius by single photon absorptiometry and the cortical area ratio (CAR) calculated from radiographs of the right hand and by x-ray of the lumbar spine. Forty-five percent of the patients showed signs of metabolic bone disease. BMC and CAR correlated with 25-OHD serum levels (P less than 0.05), especially in men. Furthermore, the amount of sun exposure has an influence not only on 25-OHD serum levels both in summer and in winter (P = 0.0006), but also on the BMC (P = 0.07). Consequently, vitamin D deficiency is of major importance for the development of metabolic bone disease in patients with Crohn's disease. Vitamin D deficiency can be prevented by increasing sun exposure and long-term vitamin D supplementation.