Carcinosarcoma of the uterus is a highly aggressive neoplasm. However, the angiogenesis of this neoplasm is still unknown. This is the first study to examine the differences in angiogenesis between the epithelial and mesenchymal elements of this biphasic neoplasm. Surgical specimens from 21 primary uterine carcinosarcomas were histopathologically evaluated, and then immunohistochemically analyzed for tumor angiogenesis, using an anti-vascular endothelial growth factor (VEGF) antibody. The microvessel density (MVD) was also measured in each element of these neoplasms, using anti-CD34 monoclonal antibody. The MVD in the epithelial element was found to be higher than that of the mesenchymal element in 20 of 21 (95.2%) primary tumors. The epithelial elements showed a higher MVD (mean, 81.6 +/- 41.1) than the mesenchymal elements (mean, 36.7 +/- 23.8) in these primary tumors (P < .0001). Moreover, the epithelial elements showed a higher VEGF expression (mean, 0.78 +/- 0.23) than the mesenchymal elements (mean, 0.37 +/- 0.20) (P < .0001). The tumors with lymph-vascular invasion showed a higher VEGF expression (n = 17; mean, 0.85 +/- 0.17) than the tumors without lymph-vascular invasion (n = 4, mean, 0.47 +/- 0.12) (P < .01). Microscopically, neither lymph-vascular space invasion nor metastatic tumors consisted of sarcoma alone in this series. In addition, a decrease in the VEGF expression was found in the transitional areas between carcinomatous and sarcomatous elements in all 10 homologous and 4 heterologous tumors evaluated. These results suggest that the tumor angiogenesis in the epithelial element may be more active than that of the mesenchymal element and also substantiated the high metastatic potential of the epithelial element in uterine carcinosarcoma. Based on these findings, carcinoma cells thus may play a key role in the angiogenesis of this biphasic neoplasm.