Inflammation of the bile ducts was studied in liver biopsies from patients with chronic hepatitis C to determine whether the frequency of inflamed bile ducts changes with therapy and correlates with other histological variables and expression of class I and II MHC antigens on ductal epithelium. Twenty patients treated at UMMC between 1991 and 1994 underwent needle biopsies of the liver before and after therapy with interferon alpha 2B (IFN). A complete response to therapy was defined as a return to normal serum alanine aminotransferase levels occurring and persisting during therapy. The number of inflamed bile ducts/total ducts (%IBDs), presence of piecemeal necrosis and lymphoid aggregates, and grade of inflammation were assessed in each high-power field in all areas with bile ducts. The frequencies of these variables were compared in cirrhotics and non-cirrhotics and in patients with complete or incomplete responses to IFN. Frozen sections of biopsies from 5 patients were immunostained using antibodies to HLA-DR and B-2 microglobulin, and positive staining was noted on bile ducts. Before therapy, the %IBD was slightly greater in patients with cirrhosis. After IFN, both %IBD and serum alkaline phosphatase levels decreased in non-cirrhotics who responded to IFN. The change in frequency of IBD with IFN paralleled the changes in the other histological features. No correlation was noted between bile duct inflammation and expression of class I and II antigens. The conclusion is that inflammation of the bile ducts occurs frequently in chronic hepatitis C, correlates with other features of inflammation in the triads, and decreases in response to IFN therapy.