Forty-three athletes under age 40, more than half of which were collegiate or professional, with partial rotator cuff tears were treated arthroscopically and observed for a minimum of 24 months. By history and mechanism of injury, two main groups were identified. Group A had 14 patients with acute, traumatic injuries. All 14 had inflamed subacromial bursas, but increased glenohumeral translation and labral lesions were uncommon. Twelve patients (86%) had satisfactory postoperative results and nine (64%) returned to preinjury sports after arthroscopic subacromial decompression and tear debridement. Group B had 29 overhead athletes with insidious, atraumatic shoulder pain. They were not as successful with debridement (19 [66%] satisfactory and 13 [45%] return to preinjury sports). Within Group B, three subgroups were identified based on the examination under anesthesia and subacromial inflammation. Group B1 (8 patients) had normal-appearing subacromial spaces and often increased anterior glenohumeral translation with posterior labral tears. These patients did poorly after arthroscopic tear debridement (3 [38%] satisfactory and 2 [25%] return to sports). Group B2 (12 patients) had inflamed subacromial bursas and increased glenohumeral translation. This group had marginal results with debridement (7 [58%] satisfactory and 6 [50%] return to sports). Group B3 (9 patients) with subacromial inflammation, yet without increased glenohumeral translation, had excellent pain relief (100%) but less than half (4) returned to preinjury sports.