This study hypothesizes that full-thickness tearing of the rotator cuff can lead to joint instability and that the degree of instability depends on the size and location of the tear. Twelve cadaveric shoulder specimens were divided into two groups: group 1 had a circular tear centered at the critical area, and group 2 had a circular tear centered at the rotator interval. Each group was tested at 2.5 cm and 5 cm tear sizes. Unloaded, and with the arm in 90 degrees flexion and full internal rotation, the humeral head shifted posteriorly. With loading, a large and more anteriorly located defect had the most influence on stability. The tear size had the greatest effect on stability in the inferior direction for group 1 and on the anterior direction for group 2. The tear location had the most significant effect on stability in the inferior and anterior directions for the smaller tear and on the anterior direction for a larger tear.