Recently, autoimmune pancreatitis (AIP) has been reported with a variety of extra-pancreatic manifestations and infiltration of IgG4-positive cells into the affected organs. We report herein two cases with prostatitis. One was seen in a patient with typical AIP, and the other was observed without any clinical manifestation of AIP. Serum IgG4 levels were elevated in both cases. Histological examination of the prostates showed that parenchymal cells were partially or totally replaced with fibrosis and abundant infiltration of IgG4-positive cells. Significant uptake of [18F] fluorodeoxyglucose by the prostate was seen in both cases. In the case with AIP, the uptake completely disappeared after steroid therapy. The findings observed in these cases suggest that the pathological mechanism for prostatitis is similar to the mechanism previously implicated in AIP, namely IgG4-related autoimmune prostatitis.