OBJECTIVES: One or more patterns of 6 K-ras mutations are detected in cells from the pancreatic juice of patients with intraductal papillary mucinous neoplasms (IPMNs). We investigated whether these mutations are associated with malignant progression. METHODS: Between January 2002 and December 2007, 53 patients with IPMN were subjected to collection of pure pancreatic juice to evaluate K-ras mutation. According to the histological and radiological findings, the IPMNs were classified into 4 groups: carcinoma group, adenoma group, high-risk group, and low-risk group. We retrospectively investigated the mutation with these groups. RESULTS: In patients with a positive K-ras mutation, a single pattern of K-ras mutation was observed in 80% (8/10) of the carcinoma group, in 71% (5/7) of the adenoma group, in 40% (2/5) of the high-risk group, and in 38% (8/21) of the low-risk group. The rate of a single pattern of K-ras mutation decreased in a stepwise order (P = 0.017). The incidence of a single pattern of K-ras mutation was significantly higher in the patients who received surgical therapy (75%, 12/16) than in those who did not (38%, 10/26; P = 0.033). CONCLUSIONS: The present study suggests that the single-clonal convergence of K-ras mutation is associated with the malignant progression of IPMNs.