In this prospective study, the authors examined 123 patients with benign or malignant neoplasms (breast cancer, n = 44; liver neoplasms, n = 43; and tumors affecting other organs, n = 36) to establish general criteria for evaluation of neoplastic lesions by means of duplex sonography. The frequency shifts determined by means of different Doppler frequencies (2.31 or 3.75 MHz) were converted into flow velocities. Only the highest systolic peak flow velocity obtained from a lesion was used for statistical evaluation. Receiver operating characteristic curves showed that a flow velocity of 0.4 m/sec is the optimal threshold value with which to differentiate benign from malignant tumors. The data obtained in all lesions indicated that only positive findings are potentially reliable. It is concluded that negative results of sonography should not be used to diagnose the presence of a benign lesion. The resistive index of the tumors was of negligible importance.