BACKGROUND: A mural nodule is a strong predictive factor for malignancy in branch duct intraductal papillary mucinous neoplasm (IPMN) of the pancreas, but the nodule size has hardly been considered. The aim of this study was to investigate whether a mural nodule of 10 mm was appropriate as an indicator of surgery for IPMN during follow-up. METHODS: The follow-up outcomes of 100 patients who had branch duct IPMN without mural nodules or who had branch duct IPMN with mural nodules of less than 9 mm in a tertiary care setting were investigated retrospectively. The patients underwent abdominal ultrasound (US) every 3 months and additional imaging examinations or cytologic examination of pancreatic juice when necessary. Surgery was recommended to them when a mural nodule developed or when a nodule enlarged and reached 10 mm. RESULTS: During an average follow-up period of 97 months, branch duct IPMNs developed mural nodules that reached 10 mm in 5 patients (0.62% per year). In one patient the IPMN was revealed to be non-invasive carcinoma by resection, 1 IPMN was shown to be malignant by further follow-up, and 3 were not resected because of refusal or the patient's age. In 7 patients, mural nodules stayed within 9 mm. The remaining 88 patients lacked mural nodules in their branch duct IPMNs throughout the follow-up. The occurrence of invasive carcinoma around the IPMN was not indicated by imaging examinations in any patient. Univariate analysis showed that the size of the cyst at baseline significantly predicted the development of a mural nodule that reached 10 mm during follow-up (P = 0.05). CONCLUSIONS: A mural nodule of 10 mm is appropriate as an indicator of surgery in the follow-up of branch duct IPMN.