After a short review of the use of ultrasonography to study shoulder rotator tendons, the technique of the investigation is described, as well as the main landmarks, findings indicative of complete rotator cuff tear, and limitations of ultrasonography in this indication. A number of issues concerning the preoperative workup are discussed. Results in a prospective study of 100 patients are reported. Findings upon ultrasonography, arthrography, and surgical exploration in these patients are analyzed comparatively. Sensitivity and specificity of ultrasonography for the diagnosis of complete rotator cuff tear (RCT) were 95% and 100%, respectively. These data show that ultrasonography is a reliable investigation for confirming, detecting, and evaluating RCT, although extensive experience with the technique is required.