Objective. Animal studies show that small intestinal bacterial overgrowth and infusion of bacterial antigens into portal blood cause hepatic histological changes similar to those seen in primary sclerosing cholangitis in man. It has been suggested that a similar mechanism involving bacterial overgrowth with increased small-bowel permeability may play a pathogenic role in patients with primary sclerosing cholangitis. Material and methods. Twenty-two patients with primary sclerosing cholangitis (13 M, 9 F, median age 37 years, range 21-74 years), 19 of whom (83%) had quiescent inflammatory bowel disease, were included in the study along with 18 healthy volunteers (9 F, 9 M, median age 36 years, range 23-80 years). Small-bowel bacterial overgrowth was denned as the presence of colonic flora > 105 colony-forming units (cfu)/ml from duodenal aspirations. Small-bowel intestinal permeability was assessed as the differential urinary excretion of lactulose/L-rhamnose. Results. Bacterial overgrowth was evident in one patient with primary sclerosing cholangitis (4.5%) (Enterobacter) and in none of the controls. Intestinal permeability in patients with primary sclerosing cholangitis (0.034 (0.026-0.041) (median, interquartile range (IQR)) did not differ significantly from that of the controls (0.033 (0.025-0.041). Conclusions. Small intestinal bacterial overgrowth and increased intestinal permeability does not seem to play an important pathogenic role in patients with primary sclerosing cholangitis. Objectives Animal studies show that small intestinal bacterial overgrowth and infusion of bacterial antigens into portal blood cause hepatic histological changes similar to those seen in primary sclerosing cholangitis in man. It has been suggested that a similar mechanism involving bacterial overgrowth with increased small-bowel permeability may play a pathogenic role in patients with primary sclerosing cholangitis. Material and methods. Twenty-two patients with primary sclerosing cholangitis (13 M, 9 F, median age 37 years, range 21-74 years), 19 of whom (83% had quiescent inflammatory bowel disease, were included in the study along with 18 healthy volunteers (9 F, 9 M, median age 36 years, range 23-80 years). Small-bowel bacterial overgrowth was denned as the presence of colonic flora> 105 Colony-forming units (cfu) / ml from duodenal aspirations. Small-bowel intestinal permeability was assessed as the differential urinary excretion of lactulose / L-rhamnose. Results. Bacterial overgro wth was evident in one patient with primary sclerosing cholangitis (4.5%) (Enterobacter) and in none of the controls. Intestinal permeability in patients with primary sclerosing cholangitis (0.034 (0.026-0.041) (median, interquartile range (IQR)) did not differ significantly from that of the controls (0.033 (0.025-0.041). Conclusions. Small intestinal bacterial overgrowth and increased intestinal permeability does not seem to play an important pathogenic role in patients with primary sclerosing cholangitis.