As an initial step in testing the hypothesis that immunoregulatory abnormalities are important in the pathogenesis of primary sclerosing cholangitis, we determined the number and percentage of lymphocyte subsets in the peripheral blood of 33 patients with primary sclerosing cholangitis. In these patients, when compared with normal and diseased controls, there was a significant reduction in the total number of circulating T cells because of a disproportionate decrease in Leu-2a (suppressor/cytotoxic) cells. This decrease resulted in a significantly increased ratio of Leu-3a to Leu-2a cells. Patients with cirrhosis had significantly higher Leu-3a/Leu-2a (helper/suppressor) ratios than did noncirrhotic patients; both disease groups, however, had ratios that were significantly higher than controls. The number and percentage of B cells were significantly increased. Alterations in the percentage of B cells correlated significantly with histologic stage and concentrations of gamma globulin, serum IgG, and bilirubin. We conclude that these abnormalities are suggestive of a defect in immunoregulation in primary sclerosing cholangitis, which is not secondary to advanced liver disease alone and appears to be independent of chronic ulcerative colitis or obstructive jaundice.