For determination of the factors that regulate biliary cholesterol secretion and the lithogenity of bile in ileal dysfunction, plasma and biliary lipids and fecal excretion of bile acids were studied in 29 patients who had undergone ileal resection. Seven patients with ileal resection had normal bile acid excretion (less than 10 mg/kg/day), and 22 had various degrees of bile acid malabsorption. None of the patients had gallstones when examined with abdominal sonography. LDL cholesterol levels were decreased in bile acid malabsorption and demonstrated a positive correlation with the molar percentage of biliary cholesterol. Biliary cholesterol (mol percent) was inversely correlated with fecal bile acid excretion. This finding suggests that biliary cholesterol secretion decreases with increasing loss of bile acids to feces in ileal dysfunction, leading to an actual decrease in the lithogenic index and to hyposaturation of cholesterol in bile. The reduction in biliary cholesterol, regarded as protecting the gallbladder mucosa against the detergent properties of bile acids, may play an important role in the pathogenesis of increased gallstone formation in ileal dysfunction.