We investigated the molecular mechanisms of angiogenesis in experimental brain metastasis. Cells from six different human cancer cell lines (proven to produce visceral metastasis) were injected into the internal carotid artery of nude mice. Colon carcinoma (KM12SM) and lung adenocarcinoma (PC14PE6 and PC14Br) cells produced large, fast-growing parenchymal brain metastases, whereas lung squamous cell carcinoma (H226), renal cell carcinoma (SN12PM6), and melanoma (TXM13) cells produced only a few slow-growing brain metastases. Rapidly progressing brain metastases contained many enlarged blood vessels. The expression of VEGF mRNA and protein by the tumor cells directly correlated with angiogenesis and growth of brain metastasis. Causal evidence for the essential role of VEGF in this process was provided by transfecting PC14PE6 and KM12SM cells with antisense-VEGF165 gene, which significantly decreased the incidence of brain metastasis. In contrast, transfection of H226 human lung squamous carcinoma cells with sense-VEGF121 or sense-VEGF165 neither enhanced nor inhibited formation of brain metastases. Collectively, the results indicate that VEGF expression is necessary but not sufficient for the production of brain metastasis and that the inhibition of VEGF represents an important therapeutic target.