The neovascularization required of fast growing malignancies in general, and ovarian cancer specifically, necessitates that they contain many new vessels that have less smooth muscle in their walls. Because of the quantitative decrease in vascular smooth muscle these vessels also have a decreased resistance to blood flow than the vessels found in benign tumors. Malignant tumors with low vascular resistance have been shown to demonstrate decreased smooth muscle actin expression and intense Cd34 expression with no differences in microvessel density (MVD) noted 8. Immunohistochemical analysis using monoclonal antibodies against smooth muscle actin (SMA) and CD34 (an endothelial cell marker) demonstrated that low resistance to blood flow in vessels within malignant ovarian tumors may be associated with a poorly developed muscular coat in the tumor vessels, compared with that observed in benign tumors. Color-flow Doppler imaging uses the altered blood flow patterns as a marker in an attempt to differentiate benign from malignant tumors 9. Researchers have evaluated the presence of intratumoral vascularization in ovarian neoplasms by Color-flow Doppler ultrasound. Benign tumors and cysts have a significantly higher pulsatility index (PI) (mean, 1.93 +/- 1.02; range, 0.23-3.99) and resistive index (RI) (mean, 0.77 +/- 0.22; range, 0.2-1.0) than do malignant tumors (PI: mean, 0.77 +/- 0.33; range, 0.31-1.09; RI: mean, 0.5 +/- 0.17; range, 0.27-0.67). Some overlap, however, exists in individual values for benign and malignant lesions. The difference in the angiogenic and vascular resistant natures of benign and malignant ovarian tumors showing intratumoral blood flow may thus be correlated with the endothelial cell activity of the tumor vessels and not the MVD. This low resistance to blood flow within tumor vessels and subsequent endothelial cell activity could be regulated by local, endothelial cell or vascular smooth muscle cell, arginine-vasopressin (AVP) synthesis and release with subsequent actions on vasopressin receptors.