Results from a clinical study are presented, in which ultrasonically-induced acoustic streaming was successfully used to differentiate fluid-filled lesions (cysts) from solid lesions in the breast. In this study, high-intensity ultrasound pulses from a modified commercial scanner were used to induce acoustic streaming in cyst fluid, and this motion was detected using Doppler methods. Acoustic streaming was generated and detected in 14 of 15 simple cysts, and 4 of 14 sonographically indeterminate breast lesions. This lesion differentiation method appears to be particularly suited for diagnosis of small, possibly newer, cysts that appear indeterminate on conventional sonography due to their size. The results indicate that this method would be a useful adjunct to conventional sonography for the purpose of breast lesion classification.