We report a 74-yr-old woman who was referred to our hospital because of abdominal fullness. ERP showed a questionable irregularity of the main pancreatic duct at the body. Examination of pure pancreatic juice was positive for K-ras point mutation at codon 12 and negative for cytology. Because neither US nor CT showed apparent lesions in the pancreas, we decided to follow up the patient with serial ERP and pure pancreatic juice studies at 3-month intervals. No changes had been seen up to 18 months later, when cytology was conclusive for malignancy with an apparent stenosis of the main pancreatic duct at the body. Distal pancreatectomy with splenectomy was performed. A round mass, 12 mm in diameter, was found in the body, which proved to be an adenocarcinoma at histological examination. No extrapancreatic extension and metastases were noted. Although positive K-ras point mutation has been reported in some cases of adenoma or mucinous cell hyperplasia of the pancreas and chronic pancreatitis, our case, along with previous reports, indicated the importance of testing K-ras point mutation in pure pancreatic juices for the diagnosis of pancreatic cancer at an early stage.