Clinical, mammographic, and sonographic preoperative size measurements were correlated with the values obtained at pathologic examination in a series of 31 patients with pathologically T1 (n = 23) and T2 (n = 8) breast carcinomas. Sonographic measurements demonstrated the highest correlation coefficient (r = 0.84), with the lowest residual standard deviation calculated in relation to the regression line. As a result, real-time sonography yielded the most accurate determination of breast cancer size in this series. Sonographic tumor size determination has proved valuable when mammography failed to delineate the tumor, particularly in patients with dense breasts. It may also be recommended in the evaluation of tumor response to nonsurgical conservative treatment.