Autoimmune pancreatitis (AIP) has been recently proposed as a disease entity, and an elevated serum IgG4 level is a characteristic finding in it. This disease is sometimes associated with other inflammatory diseases, such as retroperitoneal fibrosis and sclerosing cholangitis. To elucidate the clinicopathological characteristics of AIP-associated prostatitis (AIP-P), the clinicopathological findings of AIP-P patients were evaluated, and the immunohistochemical expression of the IgG subclasses (IgG1, IgG2, IgG3, and IgG4) in six AIP-P patients was compared with that in 10 control patients who were clinically diagnosed as suspicious for carcinoma but who had focal inflammation without adenocarcinoma on histological examination of the prostate. All AIP-P patients had the characteristic findings of AIP, and their lower urinary tract symptoms (LUTS) improved after steroid therapy. In four of five AIP-P patients, digital rectal examination indicated prostate enlargement. Histologically, AIP-P had lymphoplasmacytic and scattered eosinophilic infiltration and obliterative phlebitis accompanying gland atrophy with dense fibrosis. Immunohistochemically, the IgG4-positive plasma cell/mononuclear cell ratio was significantly higher in the AIP-P group than in the control group (P = 0.0011). AIP-P is a distinct clinicopathological entity, and a mechanism similar to that implicated in AIP may be involved in it as well.