Six hundred and eighty-one patients with ulcerative colitis who attend the outpatient clinic in Oxford have been screened for the presence of persistently abnormal liver function tests. Of the 21 patients (3.0 per cent) found with abnormal liver function 17 (2.4 per cent) were shown by cholangiography to have primary sclerosing cholangitis. The liver biopsies from those patients demonstrated a wide range of histological features and were diagnostic of primary sclerosing cholangitis in only 50 per cent of the patients. When persistently abnormal liver function tests are demonstrated in patients with ulcerative colitis it is likely that primary sclerosing cholangitis will be present (81 per cent of patients in this study), and in order to make a reliable diagnosis it is necessary to perform cholangiography in addition to liver biopsy. A close association with primary sclerosing cholangitis and histocompatibility antigens HLA B8 and DR3 is also reported.