Arthrography and surgical exploration reveal the pathogenesis of a painful shoulder syndrome characterized by inferior instability in the rotator interval (RI). The RI is the space between the subscapularis and the adjacent supraspinatus interval. Two types of RI lesions can be identified. Type I, a contracted state, is characterized by inflammatory changes in superficial bursal area. In Type II, an unstable condition is associated with extensive inflammation of deeper tissues in the RI. In a series of 101 patients, including 106 shoulders with RI lesions, surgical repair of the RI was performed with the shoulder in an externally rotated position. This position ensured a close fit of the supraspinatus and the subscapularis attachments. Of 78 shoulders evaluated in a follow-up study, 96% were graded good or excellent, and 4% poor. This investigation presents evidence for the importance of diagnosis and treatment of RI lesions in patients with persistent pain in the shoulder.