Serum lipoproteins and fecal fat were determined in 37 patients with Crohn''s disease, who were submitted to ileal resections. All patients were studied in a quiescent phase of the disease, on average 5 yr after the last operation, and all except 1 had a daily intake of dietary fat lower than 70 g. A significant positive correlation existed between the length of the resected intestine and the fecal fat excretion. Total triglyceride (TG) concentrations were similar in patients and carefully matched controls. In spite of a mean resection length of 8.6 dm of ileum, hypertriglyceridemia (type IV) was found in 7 patients. The reduced cholesterol concentration in the patients was due to a marked reduction of the low-density lipoprotein (LDL) cholesterol concentration. The LDL cholesterol concentration was negatively correlated to the length of the resected intestine. The average high-density lipoprotein (HDL) concentration was similar in healthy controls and patients, but in the latter the HDL cholesterol concentrations were lower the more extensive the ileal resections. The very-low density lipoprotein cholesterol concentration showed no significant abnormality. In patients with ileal resections other factors may compensate for the limited dietary fat supply to maintain a normal TG concentration in plasma. The low total cholesterol concentration is due to a reduction in the LDL fraction only, and probably reflects that all cholesterol used for enhanced bile acid synthesis is provided by this fraction. HDL particles may be secreted not only from the liver but also from the small intestine.