Results from both a pilot clinical study and phantom studies performed to evaluate a new ultrasonic technique to distinguish cysts from solid lesions are presented. High intensity pulses are used to induce acoustic streaming in cyst fluid, and this motion is detected using existing flow detection techniques. Since acoustic streaming cannot be generated in solid lesions, its detection would indicate a cyst. In six of seven breast cysts streaming was generated and detected in vivo. In phantom studies, a direct relationship between streaming velocity and cyst size and an indirect relationship between streaming velocity and cyst fluid viscosity were demonstrated.