PURPOSE: To assess the value of gray-scale stimulated acoustic emission in differential diagnosis between hepatocellular carcinoma and metastatic adenocarcinoma. MATERIALS AND METHODS: Twenty-four cases of epatocellular carcinoma (HCC) in 23 patients and 26 cases of metastatic adenocarcinoma in 14 patients were prospectively examined using the pulse-inversion harmonic technique after intravenous SH U 508A administration. Gray-scale stimulated acoustic emission (SAE) was measured 5 mins after bolus injection of a contrast agent (4g, 400 mg/ml). The presence or absence of SAE signals at internal and marginal areas of the tumor and the appearance (smooth or irregular) of its border were compared. In addition, the SAE index [SAE (parenchyma) - SAE (tumor)/ SAE (parenchyma)] was histographically determined using a computerized program (PiView TM ; Mediface, Seoul, Korea). The statistics were analysed using student'st test. RESULT: Of the 24 HCC cases, 20 (83%) showed internal SAE signals, while 23 (96%) marginal signals were emitted. Of the 26 cases of metastatic adenocarcinoma, one (4%) showed internal SAE signals, while in five (19%), these signals were marginal. The tumoral border was irregular in 19 HCC lesions (79%) and smooth in 23 metastatic lesions (88%). For HCC and metastatic tumors, the mean SAE index was 0.38 +/-0.15 and 0.60 +/- 0.08, respectively (p < 0.001). CONCLUSION: Gray-scale stimulated acoustic emission can be a useful tool in differential diagnosis between heatocellular carcinoma and metastatic adenocarcinoma.