Autoimmune pancreatitis (AIP) is characterized by high serum immunoglobin (Ig) G4 concentrations, lymphoplasmacytic inflammation, and a favorable response to corticosteroid treatment. Since liver dysfunction is frequently seen in AIP patients, we investigated hepatic histopathology and its clinical significance in patients with AIP. We examined the clinical features, histology, and immunoglobin G (IgG)4-bearing plasma cell infiltration of liver biopsies from 17 patients with AIP and 63 patients with either autoimmune hepatitis, primary biliary cirrhosis, primary sclerosing cholangitis, or chronic viral hepatitis and histological changes in the 7 of 17 livers before and after glucocorticoid therapy. The liver histology of AIP was classified into 5 patterns: evident portal inflammation with or without interface hepatitis (6 cases), large bile-duct obstructive features (8 cases), portal sclerosis (8 cases), lobular hepatitis (5 cases), and canalicular cholestasis (4 cases); some of the histological features coexisted in the same liver. The number of IgG4-bearing plasma cells was significantly higher in AIP patients than controls (P < 0.01), and was significantly correlated with serum IgG4 concentration (P = 0.0014, r = 0.709). Glucocorticoid therapy reduced IgG4-bearing plasma cell infiltration in the liver (P = 0.031) and ameliorated other histological findings. In conclusion, virtually all AIP liver biopsies showed evidence of various pathological changes and infiltration of IgG4-bearing plasma cells. These features were ameliorated by steroid therapy, suggesting that the liver is concurrently affected in AIP, and that liver biopsies can provide significant information in the clinical evaluation and diagnosis of AIP. (HEPATOLOGY 2007.)