The purpose of this study was to describe the cyst infection of intraductal papillary mucinous neoplasm in 2 patients. The patients were 62- and 74-year-old men. The initial symptom was acute febrile abdominal pain. Laboratory tests revealed severe infection (C-reactive protein concentrations were 23.3 µg/mL in patient 1 and 22.3 µg/mL in patient 2) and multilocular cystic masses (the diameters were 70 mm in patient 1 and 50 mm in patient 2) at the pancreatic head that involved peripancreatic vessels were demonstrated by computed tomography. Laboratory and radiographic findings were markedly improved by endoscopic transpapillary drainage. The enteric bacteria were detected in the drainage specimens. Curative resection was achieved, and histological findings indicated a carcinoma in situ in patient 1 and an invasive carcinoma in patient 2. Neither hyperamylasemia nor histological fat necrosis, frequently observed in acute pancreatitis, was evident. Both patients were free from recurrence after surgery (17 months in patient 1, and 18 months in patient 2). Cyst infection is an unknown complication of intraductal papillary mucinous neoplasm. Transpapillary drainage is highly recommended as an initial intervention. It is difficult to distinguish between cyst infection and unresectable invasive carcinoma with imaging modalities; however, surgical intervention after drainage may contribute to long-term survival.