BACKGROUND Ileostomy output and small-intestinal transport are regulated by complex mechanisms, which the present study aimed to further elucidate. METHODS The time-related ileostomy output and bile acid excretion after intraduodenal administration of a fat solution (oleic acid, 3.5 g) was studied in 29 ileostomy patients. Eighteen patients had the entire small bowel preserved (group I), whereas 11 had various lengths of the ileum resected or bypassed (group II). RESULTS Intraduodenal fat administration resulted in a prompt and significant increase in ileostomy output in both groups. The accumulated 2-h output after fat administration amounted to 60% of the normal 24-h output in group I and 30% in group II. A marked increase in bile acid excretion preceded the flow response. The fat-induced response was abolished by administration of cholestyramine. CONCLUSIONS Bile acids seem to have important regulatory effects with regard to secretion/absorption and transport of small-bowel contents, affecting ileostomy output, with clinical implications in many patients.