Bile acid absorption was studied by steady state perfusion technique in the ileum of 11 patients with regional ileitis (Crohn's disease). By computerizing absorption kinetics the presence of an active transport of glycochenodeoxycholic acid (GCDC) was rendered probable by finding a saturable transport system and a competitive absorption between conjugated bile acids. At the time of investigation 5 patients had no diarrhoea, whereas 6 patients had diarrhoea as defined from the amount of faecal output. In the former group the faecal bile acid excretion was low, the ileal absorption of GCDC high, and judged from the xylose absorption the ileal absorption surface large compared to the latter group, in which the faecal bile acid excretion was high, the ileal absorption of GCDC low, and the ileal absorptive surface small. It is concluded that malabsorption of bile acids in the ileum may be of significant physiological importance in the pathogenesis of diarrhoea in patients with regional ileitis.