AbstractBackground/PurposeSurgical indications of main duct‐involved intraductal papillary mucinous neoplasm (IPMN), especially for main pancreatic duct (MPD) of 5‐9 mm, remain controversial. We aimed to predict malignancy risk of main duct‐involved IPMN.MethodsTotal 258 patients with main duct‐involved IPMN between 2000 and 2017 in our institute were retrospectively analyzed. Main duct IPMN was classified into segmental and diffuse‐type by dilated MPD pattern. Clinicopathologic features and predictive factors for malignancy were analyzed.ResultsAmong 258 patients, 47 and 211 had pure main duct (segmental: 27, diffuse type: 20) and mixed type, respectively. Malignant IPMN presented higher in main duct type (66.0%) compared to mixed type (46.9%). The diffuse type (72.2%) had more invasive carcinoma than the segmental type (40.7%). Invasive IPMN risk increased proportionally to the MPD diameter (5 ≤ MPD <10 mm vs 10 ≤ MPD < 15 mm vs MPD ≥ 15 mm; 23.4% vs 40.0% vs 48.6%). Symptoms, elevated serum carbohydrate antigen, MPD ≥10 mm, mural nodule, thickened wall, and distal atrophy were independent predictive factors for malignancy. Patients with MPD of 5‐9 mm with at least one predictive factor had 35.0% of malignancy risk.ConclusionsThe invasive IPMN risk was different according to the dilated main duct pattern. Patients with main duct type, diffuse type, MPD ≥10 mm, and MPD 5‐9 mm with at least one predictive factor should be candidates for immediate surgery.