The aim of the study was to estimate the usefulness of measuring the circulating concentration of serum aminoterminal propeptide of type III collagen (S-PIIINP) in screening for hepatobiliary diseases in patients with ulcerative colitis. S-PIIINP was measured in 69 patients with ulcerative colitis and normal liver biochemistry, in 14 patients with ulcerative colitis and elevated catalytic concentration of alkaline phosphatases in serum (S-ALP, EC 3.1.3.1) but without primary sclerosing cholangitis (PSC), and in 20 patients with ulcerative colitis and PSC. The median S-PIIINP was 3.1 micrograms l-1 in patients with ulcerative colitis and normal liver biochemistry, 4.3 micrograms l-1 in patients with ulcerative colitis and hepatobiliary disorder other than PSC and 8.9 micrograms l-1 in those with ulcerative colitis and PSC. When the S-PIIINP cut-off level was set at 5.0 micrograms l-1, 1% of the patients with ulcerative colitis and normal liver biochemistry, 21% of those with hepatobiliary disorder, not PSC, and 90% of the patients with PSC had S-PIIINP values above that concentration. In conclusion, S-PIIINP above 5.0 micrograms l-1 in a patient with ulcerative colitis strongly suggests concomitant PSC.