High-resolution, real-time sonography of the rotator cuff was performed in 51 shoulders, and the results were correlated with findings obtained during subsequent surgery. Prospective sensitivity of sonography in detection of a tear was 100%; specificity, 75%; and accuracy, 92%. Retrospective estimation of tear size on sonograms correlated well with the intraoperative measurements for small and moderate lesions. Large lesions were often underestimated sonographically. Retrospectively, partial tears were correctly diagnosed in seven patients, and bursal thickening was recognized in 17 patients. Sonography of the shoulder proved to be an accurate, noninvasive method for the diagnosis of complete rotator cuff tears. It is useful for estimating tear size and location and may be useful in recognizing partial tears.