Angiogenesis is crucial for tumor growth and metastasis. Hepatocellular carcinoma (HCC) is a typical hypervascular tumor. However, the relationship between tumor vascularity and the outcome of patients with HCC has not been evaluated. To clarify whether tumor angiogenesis is related to the prognosis of patients, immunohistochemical staining, using anti-von Willebrand factor (vWF) and anti-CD34, was applied in resected specimens from 43 cases of HCC. In nonmalignant tissue, staining was confined to vessels in the portal tract and to a few periportal sinusoids with both of the endothelial markers applied. In tumor tissue, however, sinusoid-like vessels reacted intensively with anti-CD34 but not with anti-vWF. The intratumor microvessel density (MVD) highlighted by anti-CD34 was 297 +/- 88 (per 0.74 mm2), which was significantly higher than that highlighted by anti-vWF (4 +/- 7). When only the MVD highlighted by anti-CD34 was analyzed, tumor diameter larger than 2 cm, poor differentiation (Edmondson's II to IV), and portal invasion were significantly related to the subgroup with MVD > or = 290. Overall survival curves of patients with MVD < 290 were better, and these patients were more likely to remain tumor free. Cox hazards model revealed intratumor MVD and Edmondson's grade to be independent prognostic factors for the overall survival of patients. These results demonstrated for the first time that tumor angiogenesis assessed by anti-CD34 was correlated with the outcome of patients with HCC, suggesting a potential role for anti-CD34 in the diagnosis and treatment of HCC.