PURPOSE: The purpose of this study was to evaluate the efficacy of transvaginal color Doppler sonography (TVCDS) in the diagnosis of retained placenta. MATERIALS AND METHODS: 24 cases pathologically diagnosed retained placenta and underwent TVCDS before treatment were included in this study. The clinical findings, TVCDS findings, and pathological findings were reviewed retrospectively. We evaluated size, location, echo character, and myometrial invasion with gray scale. Presence, grade (1-3), location, peak systolic velocity (PSV), and resistive index (RI) of blood flow in retained placenta were observed. RESULTS: In transvaginal sonography, 21 cases had cystic changes and heterogeneous echo texture in retained placenta. The sizes of retained placenta were 1.4~6.4 cm (mean 3.4 cm), and 17 cases demonstrated myometrial invasion. In TVCDS, 19 cases showed blood flow in retained placenta but 5 cases did not. Most blood flows were grade 2 and located in either periphery or both periphery and center of the mass. PSV were 15.4~99.3 cm/sec (mean 47.33 +/-22.75cm/sec) and RI were 0.17~0.43 (mean 0.31 +/-0.07). RI of blood flow inretained placenta negatively correlated with the size (p=0.033<0.05). CONCLUSION: Retained placenta demonstrated with TVCDS shows blood flow of high velocity and low impedance. TVCDS may be a useful tool in the diagnosis of retained placenta.