PURPOSE: To evaluate the efficacy of color Doppler ultrasound (US) in the diagnosis of gestational trophoblastic neoplasia (GTN). MATERIALS AND METHODS: Intralesional color flows and resistive index (RI) on color Doppler US were prospectively analyzed in 21 consecutive suspected GTN cases. RI of the intralesional artery was investigated on the basis of the presence or absence of mass and metastasis. Correlation between RI of intralesional artery and urinary beta-hCG was also investigated. RESULTS: Intralesional color flows were identified in 15 patients with GTN. On operation, intralesional color flows were observed in one of two patients in whom the presence of completely necrotic tissue was confirmed. Intralesional color flows, however, were not detected in four patients who were proved not to be GTN sufferers. Sensitivity, specificity, accuracy, positive and negative predictive values, and accuracy were 100%, 83%, 95%, 94% and 100%, respectively. Significant correlation between RI of the intralesional artery and urinary beta-hCG was not established(p=0.49, r=0.19). RI of this artery was not substantially different between groups with and without mass, and between groups with and without metastasis(p=0.32, p=0.82). CONCLUSION: The current study demonstrates that color Doppler US is a sensitive and useful method for the diagnosis of GTN.