A detailed anatomic study of 123 shoulders was performed to define the location of the acromioclavicular (AC) joint within the supraspinatus outlet and to determine the correlation of AC joint morphologic characteristics with the presence of full-thickness rotator cuff tears (RCTs). The presence, location, and extent of RCTs were first documented for each shoulder, and 2 anatomic landmarks consisting of the anterolateral acromial corner and the inferior midpoint of the AC joint were identified and their positions digitized in 3 dimensions to calculate outlet space and clearance and AC joint locations. High resolution x-ray films were then obtained to determine the extent of degenerative changes including the location and size of inferior osteophytes and joint space. These specimens were cleaned of all soft tissues, and additional measurements were made. All data were entered into a computerized database and analyzed with respect to age, sex, and the presence of RCT. Comparison of age-matched RCT versus non-RCT groups revealed no significant differences in the position of the AC joint within the supraspinatus outlet. When AC joint morphologic characteristics were compared with those of a nontear, age-matched group, no significant differences with regard to joint space narrowing were found, but significantly larger and greater number of osteophytes were identified on both sides of the joint in the RCT group. In conclusion, the position of the AC joint is fairly constant within the supraspinatus outlet and does not appear to predispose to RCTs. The morphology of the AC joint contribution of the tears seems to be acquired in nature rather than inherent in its location within the outlet.