Purpose : We assessed the usefulness of resistive index(RI) on spectral analysis of Doppler sonography fordifferential diagnosis of benign and malignant breast lesions. Materials and Methods : We retrospectively reviewed29 benign and 22 malignant lesions of breast, which were examined preoperatively with color and duplex Doppler andwere confirmed by histopathologically after operation. We analyzed the average and distribution of RI in benignand malignant lesions. Results : Although, there was no difference in the average values of RI in benign andmalignant breast lesions, the distribution of RI was below 0.7 in eighteen cases (62%) of benign lesions, andabove 0.7 in eighteen cases (82%) of malignant lesions. Thus, RI is valuable for differentiation between benignand malignant lesions of breast. Conclusions : Measurement of RI in breast disease using color and duplex Dopplerstudy is useful modality adjunct to the conventional ultrasonographic differentiation of benign and malignantlesions.