Background: Monitoring of gene alterations in blood to track circulating tumor DNAs has been attempted for a clinical application. For example, KRAS monitoring in colorectal cancer provides a valuable biomarker for diagnosis and prediction of treatment outcome. While half of of colon cancer has RAS mutation, 90% of pancreatic cancer shows KRAS mutation, suggesting that most of pancreatic cancer is a good candidate for KRAS monitoring. In this study, we elucidated the clinical significance of KRASmonitoring in patients with pancreatic tumor during treatments. Methods:KRAS mutation in tumor tissues was determined by Scorpion ARMS or RASKET methods in 39 patients with pancreatic tumors. KRASmutation in blood (G12D, G12V, G12C, G12A, G12S, G12R, G13D, Q61L, Q61H) were investigated by using droplet digital polymerase chain reaction (ddPCR) in 24 patients treated with chemotherapy. Results:KRAS assessment in tumor tissues showed 33 patients with mutation and 6 patients without mutation. Thirty-three patients with mutation showed significantly poor prognosis of 49% in three years overall survival (OS) as compared with 100% in 6 patients with mutation (p=0.02). KRAS assessment in blood revealed that KRAS mutation was detected in 14 patients, but no detection was seen in 10 patients. Patients harboring KRAS mutation in blood exhibited significantly poor treatment outcome, including 12 patients with progressive disease, as compared with 10 patients without detection of mutation, including 6 patients with any treatment responses (stable disease in 4 patients and partial response in 2 patients, p=0.03). Fourteen patients with mutation in blood displayed poor prognosis of 20% in three years overall survival (OS), comparing to 69% in 10 patients without mutation in blood (p=0.06). Conclusions:KRAS status in tumor tissues was involved in prognosis; in addition, KRAS status in blood was implicated in treatment response of chemotherapy in patients with pancreatic tumor. KRAS monitoring in tumor tissues and blood provides useful information for the treatment strategy including chemotherapy in patients with pancreatic tumor.