In this study the clinical usefulness of ultrasonography in the diagnosis of rotator cuff tears was assessed. One hundred twenty-two patients undergoing preoperative ultrasonography were evaluated surgically. Three different ultrasonographic machines were applied with a 7.5-MHz linear-array transducer. Surgery confirmed a sensitivity of 79% for full-thickness tears and 50% for partial thickness tears. In 73 patients with negative ultrasonographic findings, the specificity was 100% for full-thickness tears and 90% for partial-thickness tears. The diagnostic sensitivity for full-thickness tears of the Hitachi EUB-340 system was significantly lower than those of Hitachi EUB-415 and EUB-515, and thus the former cannot be recommended for detection of rotator cuff tears. The sensitivity with regard to partial-thickness tears was also significantly lower with the Hitachi EUB-340 system as compared to the Hitachi EUB-515. There were no significant differences in false-positive rate among the three machines used. In conclusion, accurate ultrasonographic diagnosis of rotator cuff tear requires a high-resolution machine, as well as considerable experience in interpretation.