This study explored decreased tumor suppressor microRNA (miRNA) plasma levels in pancreatic cancer (PCa) patients to clarify their potential as novel biomarkers and therapeutic targets. We used the microRNA array-based approach to select candidates by comparing plasma levels between PCa patients and healthy volunteers. Six down-regulated miRNAs (miR-107, miR-126, miR-451, miR-145, miR-491-5p, and miR-146b-5p) were selected. Small-and large-scale analyses using samples from 100 PCa patients and 80 healthy volunteers revealed that miR-107 was the most down-regulated miRNA in PCa patients compared with healthy volunteers (P < 0.0001; area under the receiver-operating characteristic curve, 0.851). A low miR-107 plasma level was significantly associated with advanced T stage, N stage, and liver metastasis and was an independent factor predicting poor prognosis in PCa patients (P = 0.0424; hazard ratio, 2.95). miR-107 overexpression in PCa cells induced G1/S arrest with the production of p21 and inhibited cell proliferation through the transcriptional regulation of Notch2. In vivo, the restoration and maintenance of the miR-107 plasma level significantly inhibited tumor progression in mice. Depletion of the tumor suppressor miR-107 in plasma relates to tumor progression and poor outcomes. The restoration of the plasma miR-107 level might be a novel anticancer treatment strategy for PCa.Pancreatic cancer (PCa) is the fourth leading cause of cancer-related death in Japan and the United States 1 and the seventh worldwide 2 . PCa is one of the most aggressive cancer types and constitutes a global health problem, with more than 330,000 cancer-related deaths annually 2 . Although perioperative chemo-and/or radiotherapy regimens, surgical techniques and perioperative management have greatly progressed, PCa continues to present an extremely poor prognosis. Even now, the median survival time of PCa patients is 5 to 8 months, and their 5-year survival rate is less than 10%. This is because PCa develops with no symptoms, local invasiveness, or metastases to distant organs in the early stage of its clinical course [3][4][5] . Therefore, novel early diagnostic tools and effective treatment strategies are urgently needed to improve the survival rate of PCa patients.Because understanding the molecular mechanisms of tumorigenesis and identifying clinical biomarkers and molecular targets for PCa contribute to improving the management of this lethal disease, several studies have attempted to detect the biological factors involved in the malignant potential of PCa 6, 7 . Nevertheless, in clinical settings, no molecule has been used as an early diagnostic biomarker, and only a few molecules have been