OBJECTIVE The purpose of this study was to determine the diagnostic value of the sonographic halo sign (defined as any hypoechoic rim in the periphery of a lesion) in distinguishing between benign and malignant isoechoic and hyperechoic liver lesions on sonography. METHODS Sonograms of the liver in 50 patients with proved benign liver tumors and in 50 patients with proved malignant liver tumors (seven primary liver neoplasms, 43 metastases) selected during a 13-month period were retrospectively analyzed by four radiologists who had no knowledge of the patients' clinical findings or the final diagnoses. Only a single sonogram was studied in each case. The presence or absence of a hypoechoic halo on the sonogram was the only criterion for distinguishing malignant from benign hepatic lesions. RESULTS For 95 of 100 hepatic lesions, the four radiologists were almost (three vs one) or completely (four vs zero) in agreement about the presence or absence of a hypoechoic halo. In the five cases where there were conflicting decisions (two vs two), a final decision (four vs zero) was achieved by reviewing the entire series of sonographic images. A halo could be detected in 44 malignant tumors (88%) and in only seven benign tumors (14%) (sensitivity, 88%; specificity, 86%; positive and negative predictive values, 86% and 88%, respectively). The sonographic halo sign was particularly helpful in distinguishing hemangiomas (n = 29) from metastases (n = 43) (positive and negative predictive values, 95% and 87%, respectively). CONCLUSIONS The results of this study suggest that the halo sign on sonograms is useful to distinguish benign from malignant isoechoic or hyperechoic tumors.