Purpose. The aim of this study was to evaluate the diagnostic accuracy and clinical value of preoperative sonographic detection of multifocality in breast carcinoma. Methods. Between November 1, 1999, and November 30, 2000, we prospectively compared the sonographic and histopathologic findings for all nodules measuring 1 cm or less in the longest dimension on sonography in 19 consecutive patients diagnosed with breast carcinoma. For each nodule, the final histopathologic diagnosis, size, and, in cases of nodules in the same breast as the primary lesion, the distance from that lesion were evaluated. Results. The 19 patients examined, all of whom were women, had a mean age of 42.1 years (range, 28-62 years). Thirty nodules were detected on sonography: 15 patients had a total of 24 nodules in the ipsilateral breast, 2 patients each had 1 nodule in the contralateral breast, and 2 patients each had 1 nodule in the ipsilateral breast and 1 nodule in the contralateral breast, for a total of 26 ipsilateral and 4 contralateral nodules. Of the 30 lesions, 20 (67%) appeared to be malignant (ie, satellite nodules) on sonography. Histopathologic examination showed that of these malignant-looking lesions, 15 (75%), which were found in 10 patients, were malignant and 5 (25%), which were found in 4 patients, were benign. For 7 (70%) of the 10 patients whose nodules appeared malignant on sonography, the initial treatment plan was changed because of the sonographic findings. Ten lesions were assessed as benign on sonography; histopathologic examination confirmed that all 10 were benign. Sonography had a sensitivity of 100%, a specificity of 67%, a positive predictive value of 75%, a negative predictive value of 100%, and an accuracy rate of 83%. The mean size of the nodules was 0.8 cm (range, 0.3-1.4 cm). The mean distance of the 26 ipsilateral nodules from the primary lesion was 1.8 cm (range, 0.5-6 cm). Conclusions. Preoperative sonography is accurate and reliable for detecting small satellite nodules and can have an important role in the preoperative evaluation of patients for whom breast-conserving surgery is planned. © 2003 Wiley Periodicals, Inc.