The effects of type II superior labrum anterior-posterior (SLAP) lesions on glenohumeral rotation and translation were studied in 6 cadaveric shoulders before and after repair. Glenohumeral translation with the application of 15 N and 20 N in the anterior, posterior, superior, and inferior directions was measured with the joint in 60 degrees of abduction and 90 degrees of external rotation. Data were recorded for intact shoulders, shoulders with arthroscopy portals, shoulders with arthroscopically created anterior type II SLAP lesions, shoulders with arthroscopically created anterior and posterior type II SLAP lesions, and shoulders that had undergone arthroscopic repair. With the creation of a SLAP lesion, significant increases in total range of motion (P = .028), external rotation (P < .0001), internal rotation (P < .01), anterior-posterior translation (P < .0001), and inferior translation (P < .01) were observed. After arthroscopic repair, total range of motion, internal rotation, external rotation, and translation significantly decreased, returning to expected values. These findings suggest that type II SLAP lesions cause significant glenohumeral instability, which can be effectively treated with current arthroscopic techniques.