Intestinal absorption of fat and vitamin B12 was studied prospectively in 136 patients with Crohn's disease localized to the ileum and/or the colon. Fecal fat was measured in 3-day periods on a high fat diet, and vitamin B12 absorption was assessed by the Schilling test. When related to the extent of the ileal disease there was a poor correlation to both fecal fat excretion and the result from the Schilling tests, but an exponential and highly significant correlation was found between the results of both tests and the length of the resected ileal segment. Accepting a change in either direction not exceeding 10% of the preoperative test value as unchanged, three-fifths of the patients deteriorated and one-fifth improved in fat-absorption, whereas half the patients deteriorated and one-third improved in vitamin-B12 absorption after surgery. When related to the length of the ileum resected, the test results were about the same in patients subjected to a restorative procedure sparing the major part of the colon and in ileostomy patients. Absorption of fat and vitamin B12 did not change with time elapsing from operation, indicating that there were no compensatory mechanisms as regards absorption of these substances.