The histopathologic changes leading to rotator cuff rupture are gradual and progressive. Incomplete tears can be observed in the articular or bursal surface. These partial lesions are infrequently demonstrated with arthrography or bursography. Although structural variations in cuff integrity can be demonstrated with ultrasound or magnetic resonance imaging, precise definition of partial tears is difficult. The exact location and extent of incomplete tears can be documented with shoulder arthroscopy. Anterior acromioplasty, either open or arthroscopic, is indicated for the treatment of chronic mechanical impingement refractory to conservative management. When a sizable partial defect is identified at open surgery, the degenerated tissue is excised and the tendon is reattached to bone or repaired with side-to-side suture. Arthroscopic treatment consists of debridement of the torn cuff margins, followed by arthroscopic subacromial decompression (ASD). When the incomplete tear in an active individual involves more than one-half the cuff thickness, arthroscopic and open techniques can be combined. Twenty partial-thickness tears were encountered among 130 patients who had ASD for chronic impingement. Fifteen had a satisfactory result, but five required additional surgery. A system of grading partial-thickness tears based on location, depth, and area is presented in an effort to standardize the observations of various investigators and to permit comparison of the results of arthroscopic treatment.