The prevalence of primary sclerosing cholangitis (PSC) in patients with ulcerative colitis (UC) attending the Depts. of Medical and Surgical Gastroenterology, Aalborg Hospital, during a 12-year period, was determined. All patients with an alkaline phosphatase (ALP) value above the normal range were investigated. Of 305 patients with UC, 24 patients had elevated ALP values, and 11 of these (3.6% of the study population), 4 males and 7 females, were found to have PSC by direct cholangiography. In five patients the disease worsened (two patients died of cholangiocarcinoma), in four it was stationary, and in two patients the disease improved during a mean observation period of 6 years. No differences in location of disease, disease activity, or duration of disease were found between patients with UC and PSC and patients with UC without PSC. The ALP values were raised to a mean of 3.7 times the upper normal limit (observed range, 1.5-5.5 times the upper normal limit). Aspartate aminotransferase was moderately elevated in most patients, but no other abnormal biochemical liver test results were observed at onset. The results of our study indicate that PSC is the major cause of raised ALP values in patients with UC; thus cholangiography should be performed in UC patients with unexplained elevated ALP levels. A prognostic indicator is needed to predict the individual prognosis and to determine the optimal timing of liver transplantation.