Severe histological osteomalacia developed in a woman with Crohn's disease 2 years after ileal resection and the start of cholestyramine therapy. Treatment with oral 25-hydroxyvitamin D3, 50 microgram daily, produced marked biochemical, radiological, and histological improvement after 6 months. It is suggested that cholestyramine, by reducing vitamin D absorption, precipitated the rapid development of osteomalacia in this patient. This case report emphasizes the importance of routine vitamin D supplementation in all patients on long term cholestyramine therapy, and indicates that even in patients with small intestinal disease or resection, oral preparations of vitamin D or its metabolities and analogues may be effective.