Angiogenesis is necessary for growth and the spread of human tumors. Animal studies also suggest that angiogenesis is an important interspecies biological mechanism of tumor development. Angiogenesis is a complex multistep cascade modulated by both positive soluble factors, such as vascular endothelial growth factor, thymidine phosphorylase, basic-fibroblast growth factor and negative soluble factors such as angiostatin and endostatin. From the imbalance of the above angiogenesis regulators, tumor endothelial cells may divide up to 50 times more frequently than endothelial cells of normal tissue. Published studies have suggested that the assessment of microvessel density (MVD) or endothelial area (EA) can be considered as surrogate markers of angiogenesis with biological and prognostic relevance. Literature data on angiogenesis of mesothelioma are inconclusive, with only a few studies performed in primary peritoneal mesothelioma (PPM) due to the rarity of the disease. We assessed immunohistochemically MVD and EA and their biological and clinical significance in a consecutive series of 23 PPM cases. MVD and EA were detected in “hot spots” by a computerized image analyzer. The mean value of MVD and EA was 27 ± 14 and 26.04 ± 8.35 x10-2 μ2 per field (400x), respectively. Patients with a high MVD or EA tumors showed a more clinical aggressiveness due to the presence of ascites and a shorter overall survival. Our results suggest that PPM is an angiogenesis-dependent neoplasia. Therefore, antiangiogenic compounds should be tested particularly in those patients with highly vascularized PPM.