A 78-year-old man presented with upper abdominal pain. He underwent an abdominal computed tomography scan which revealed irregularly shaped mass lesions in the mesentery and in the pancreatic head and body. The mass lesions were suspected to be pancreatic cancer with peritoneal dissemination and a surgical biopsy was performed. Histologic studies revealed lymphoplasmacytic sclerosing pancreatitis with significant infiltration of IgG4-positive plasma cells. His serum IgG4 level was 223 mg/dl. Findings from histologic and serum studies were compatible with IgG4-related sclerosing disorders. The mass lesions responded to steroid therapy and disappeared. The difficulty in making a definitive diagnosis is discussed.