Tumor thrombosis of the portal vein was identified retrospectively with computed tomography (CT) in four patients aged 66-77 years with gastric adenocarcinoma. Surgical, clinical, histopathologic, laboratory, and imaging findings were analyzed. Three patients showed an elevated alpha-fetoprotein (AFP) level (230-1,560 ng/mL [230-1,560 micrograms/L]). Immunohistochemical study revealed that AFP was produced by gastric carcinoma in two patients. Multiple metastatic foci in the liver appeared on CT and ultrasound (US) scans in all four patients. Echogenic thrombus was identified in three. There were no CT or US features that enabled differentiation of neoplastic from nonneoplastic thromboses. Angiography showed tumor vessels in only one patient: The thrombus was hypervascular in the arteriocapillary phase of celiac angiography but could not be differentiated from a much more common tumor thrombus seen in hepatocellular carcinoma. Nevertheless, gastric carcinoma should be considered a possibility in the diagnosis of portal venous tumor thrombosis, even if the serum AFP level is elevated and a liver tumor is identified.