Differentiation of breast cancer has been receiving increasing attention in attempts to minimize the numbers of excisional biopsies for benign tumors. Doppler spectral parameters, such as peak systolic velocity (PSV) and pulsatility index (PI), have been used but these are unable to differentiate breast cancers and benign tumors with adequate levels of specificity and sensitivity. The aim of this study was to determine whether the acceleration time index (ATI) could reliably differentiate breast tumors. In 102 breast masses, Doppler ultrasonography (US) was performed with a LOGIQ700MR. PSV, PI and ATI were calculated from intratumoral flow. Forty-eight malignant tumors were identified by histologic evaluation, 13 benign tumors were diagnosed by fine needle aspiration biopsy (FNAB) and 41 benign masses were diagnosed by characteristic gray-scale findings and long-term (>18 months) follow-up examinations. Doppler signals were detected in 35 of 54 (64.8%) benign lesions and in 46 of 48 (95.8%) breast cancers. PSV and PI value showed no statistical significance between malignant and benign lesions (PSV; p=0.1147, PI; 0.1506). ATI values were lower in benign lesions (p<0.0001). An ATI value of 0.14 seems to be the optimum threshold for the differentiation of benign and malignant lesions (79.4% sensitivity, 87.2% specificity). No malignant lesions were seen when the ATI value was <0.14 and the PI value was <1.1. ATI was found to be the most useful parameter for differentiating benign breast tumors from malignant ones.