To clarify the role of fat maldigestion in the pathogenesis of steatorrhea in patients with ileal resection the total and aqueous phase concentrations of bile acid and fatty acid were characterized in 8 such patients (5 patients with small ileal resection, bile acid diarrhea, and steatorrhea less than 20 g per day; 3 patients with large ileal resection, fatty acid diarrhea, and steatorrhea greater than 20 g per day) as well as 4 healthy control subjects after a morning and an afternoon liquid test meal. The study was then repeated with cholestyramine, 4 g being administered before each meal to induce fat maldigestion. After a conventional test meal, patients with large resections and severe steatorrhea had significantly lower aqueous phase concentrations of bile acids (and fatty acids) than patients with smaller resections or control subjects, explained in part by intraluminal precipitation of about one-half of the bile acids during digestion. When cholestyramine was administered before the meal, aqueous phase bile acid concentrations decreased in all patients, including the normal control subjects; the degree of fat maldigestion induced in the patients with small resections (and the control subjects) became similar to that present after the conventional test meal in the patients with large resections. Because steatorrhea increased little in the patients with small resections when cholestyramine was administered continuously, the data suggest that fat maldigestion per se does not induce severe fat malabsorption in patients with sufficient anatomical reserve, because such patients can absorb fat efficiently by utilizing the distal small intestine. In patients with large ileal resections, severe steatorrhea is explained in part by the combination of fat maldigestion and decreased surface area. It is also speculated that the steatorrhea occurring in patients with small resections and relatively normal fat digestion during two test meals may be explained by impaired fat digestion which occurs during the final meal of the day, which is often the largest meal.