BACKGROUND The portal vein thrombosis caused by a carcinoma of the pancreas or liver could be a diagnostic challenge. There are some cases which could be diagnosed only by observing changes in patients' clinical manifestations or imaging studies over time. CASE REPORT A 63-year-old man experienced exacerbation of chronic low back pain for 1 month, with abdominal distention. He was admitted to our hospital because abdominal ultrasonography and abdominal contrast-enhanced computed tomography performed at another hospital revealed an extensive thrombus in the portal, splenic, and superior mesenteric veins. Anticoagulation therapy with warfarin was started immediately on admission. Although no baseline disease, such as malignancies, coagulopathies, infections, or collagen diseases were revealed during the 12 days of his hospitalization, the thrombus gradually expanded, with the appearance of obstructive jaundice 8 months after admission. He was readmitted at that time for biopsy of intrahepatic tumors detected by abdominal computed tomography, which diagnosed metastasis of a carcinoma of unknown origin. Although chemotherapy was started, he died 11 months after his initial admission to our hospital. Autopsy revealed a tumorous lesion in the pancreatic head with almost complete replacement of the portal venous thrombus with a similar-appearing tumor. We confirmed the diagnosis of portal venous metastatic infiltration by undifferentiated pancreatic carcinoma, using histopathological examinations. CONCLUSIONS It is essential to consider portal venous metastatic infiltration of undiagnosed malignancy inducing local hypercoagulopathy as a possible cause of expansive portal venous thrombosis.