We have generated a transgenic mouse where hVEGF-A₁₆₅ expression has been silenced with loxP-STOP fragment, and we used this model to study the effects of hVEGF-A₁₆₅ over-expression in mice after systemic adenovirus mediated Cre-gene transfer. Unlike previous conventional transgenic models, this model leads to the expression of hVEGF-A₁₆₅ in only a low number of cells in the target tissues in adult mice. Levels of hVEGF-A₁₆₅ expression were moderate and morphological changes were found mainly in the liver, showing typical signs of active angiogenesis. Most mice were healthy without any major consequences up to 18 months after the activation of hVEGF-A₁₆₅ expression. However, one mouse with a high plasma hVEGF-A₁₆₅ level died spontaneously because of bleeding into abdominal cavity and having liver hemangioma, haemorrhagic paratubarian cystic lesions and spleen peliosis. Also, two mice developed malignant tumors (hepatocellular carcinoma and lung adenocarcinoma), which were not seen in control mice. We conclude that long-term uncontrolled hVEGF-A₁₆₅ expression in only a limited number of target cells in adult mice can be associated with pathological changes, including possible formation of malignant tumors and uncontrolled bleeding in target tissues. These findings have implications for the design of long-term clinical trials using hVEGF-A₁₆₅ gene and protein.