Thyroid lesions are a common observation in ultrasound examination. In most cases the differentiation of malignant and benign tissue is difficult. In this study 133 thyroid lesions were examined by an ultrasonographic tissue characterization system prior to surgery. The pathologic findings (29 cancers, 31 adenomas, 46 nodular goiters, 27 cysts) were examined with reference to the quantitative evaluation of the ultrasonic images (ROI method). The selected regions of interest (ROI) were analyzed with regard to the three most discriminating parameters. The overall accuracy of differentiation between malignant and benign thyroid nodules was 83.9%.