The aim of the present study was to find a possible relationship between biological behavior of breast cancer and sonographically detectable blood flow. 157 patients with ductal invasive breast cancer were examined by means of a new sonographic procedure, the MEM (Maximum Entropy Method), able to detect considerably lower blood flow velocities than doppler sonography. In the absence of objectifyable quantification methods, findings were allotted to 3 classes, depending on the visual color information obtained. Blood flow was correlated to tumor size, lymph node and receptor status, ploidy and S-phase-fraction. Most patients with small tumors, no lymph node metatases, positive receptors, diploid genome and low S-phase found to have low blood flow. This close relation between established prognostic factors and results of sonographic blood flow examination with the MEM might indicate a new preoperative prognostic factor, which, however, will have to be proven by larger studies.