The prevalence of vitamin D deficiency in Crohn's disease and the relationship of vitamin D status to metabolic bone disease have not been fully characterized. Serum 25-hydroxyvitamin D was measured in 82 patients with Crohn's disease; 65% of Crohn's disease patients had a low serum 25-hydroxyvitamin D concentration; 25% had deficient levels (<10 nglml). The lowest 25-hydroxyvitamin D levels were observed in patients with previous ileal resections. Nine patients were studied in detail including transiliac needle bone biopsies; 6 had osteomalacia and 3 osteoporosis. Six patients had repeat bone biopsies 9 to 18 mo after vitamin D treatment. Three patients with osteomalacia and low serum 25-hydroxyvitamin D levels showed histologic improvement after therapy with oral vitamin D restored serum 25hydroxyvitamin D levels to normal. The adequacy of therapy was assessed accurately by monitoring serum 25-hydroxyvitamin D concentration. Three patients with metabolic bone disease with normal serum 25-hydroxyvitamin D levels at diagnosis did not show histologic improvement after receiving vitamin D.