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[Objective] Intraductal papillary mucinous neoplasms (IPMNs) of the pancreas are mainly divided into branch duct type and main duct type, and malignant potential of main duct type is considered to be higher than that of branch duct type. International guidelines recommend resection of all main duct IPMNs and branch duct type having cyst size greater than 30 mm, mural nodule, dilation of main pancreatic duct, or symptoms such as pancreatitis. Combined type IPMNs are also indication for surgical resection because this type is considered to have the same malignant potential as the main duct type; however, biological behavior of combined type IPMNs has not been well understood. Moreover, definition of combined type IPMNs seems to be different from institution to institution, and whether combined type might be derived from branch duct or main duct IPMN has not been well elucidated. The aim of this study was to investigate the malignant potential and prognosis of combined type IPMNs defined by our criteria using preoperative imaging study.[Methods] We considered that combined type IPMNs result from tumor spread of branch duct IPMNs to main pancreatic duct, and therefore, defined the tumor with cystically dilated branch(es) and main pancreatic duct dilatation (more than 5mm) as combined type. The study population included 177 IPMNs resected at our institution. There were 34 main duct type (23 carcinomas), 62 branch duct type (13 carcinomas), and 81 combined type (26 carcinomas). The frequency of malignant IPMNs and prognosis were compared between each type. [Results] The incidence of malignant IPMNs in main duct type (68%) was significantly higher than that of branch duct (21%), or combined type (32%) (p<0.05). On the other hand, 10-year disease-specific survival rates of branch duct type, main duct type, and combined IPMNs were 64%, 61%, and 66%, respectively. Even when focusing on invasive IPMNs, the 10-year disease-specific survival rate were not different in branch-duct (22%), main duct type (39%), and combined type (23%) (p=0.4).[Conclusion] Biological behavior of combined type IPMNs seems to be similar to branch duct type IPMNs; however, such a result might be caused by our definition criteria and underestimation of the malignant change of branch duct IPMN based on the selection of patients for resection. Further investigation using other definition criteria of combined type IPMNs might be necessary.