We compared the intestinal absorption of cholecalciferol and 25-hydroxycholecalciferol in patients with Crohn's disease and resections of the small bowel. Patients were subgrouped into those with small (< 100 cm), intermediate (100-300 cm), and large (> 300 cm) resections. 3Hcholecalciferol or 3H25-hydroxycholecalciferol were given orally and serial blood samples were taken for measurement of plasma radiolabeled vitamin. Absorption of both forms of the vitamin decreased with extent of resection but 25-hydroxycholecalciferol absorption was always greater than that of cholecalciferol. When compared with normal control subjects, 25-hydroxycholecalciferol absorption in these patients was better maintained than that of cholecalciferol. These data indicate that vitamin D malabsorption reflects the extent of distal small-bowel resection in Crohn's disease. Treatment with oral cholecalciferol is sufficient in those with small or moderate resections but oral 25-hydroxycholecalciferol supplementation may be preferred in those with a severe short-bowel syndrome.