In this report we describe a metastatic monkey hepatocellular carcinoma (HCC) model in which intravascular metastatic development is clearly evident. The tumor-bearing livers contained intravenous tumor thrombi at different stages of progression within the small branches of the portal vein. These ranged from mural tumor thrombi lined with CD31-positive endothelial cells to tumor thrombi that had completely occluded the vascular lumen. Intravenous tumor expansion was frequently accompanied by the appearance of CD31-positive microvessels within the tumor thrombi and fibrous perivascular thickening, giving rise to isolated tumor nodules within the portal areas. Intravascular expansion of disseminating HCC was also evident within small branches of the pulmonary arteries in the lungs. These findings indicate that metastases of HCC can become established while still at an intravascular stage, suggesting that the direct interaction between tumor cells and parenchymal cells predicted from experimental rodent metastasis models is not a prerequisite for the metastatic development of these tumors.