A replacement arthroplasty was performed in 236 shoulders at The Hospital for Special Surgery from 1984 through 1989. Ten patients (eight women and two men) from that group were identified as having instability of the shoulder at the time of follow-up, and the results for these patients were reviewed retrospectively. The ages of the patients ranged from fifty-six to seventy-nine years. The instability was anterior in seven of the patients and posterior in three. The anterior instability was caused by a rupture of the repaired subscapularis tendon. The operative treatment of the anterior instability consisted of mobilization and repair of the tendon, but three of the seven patients continued to have instability. A static stabilizer, consisting of an allograft of Achilles tendon, was inserted in these three patients, and the result was a success. The etiology of the posterior instability (three patients) was multifactorial. Treatment consisted of correction of any soft-tissue imbalance and revision of the prosthetic components as necessary. All ten patients were followed clinically and radiographically for at least two years. All of the patients had some loss of motion of the shoulder as compared with the motion before the dislocation. There were no neurovascular complications, problems related to the allografts, or any other complications. We concluded that proper balancing of the soft tissues and positioning of the prosthetic components are essential to a successful arthroplasty of the shoulder. The postoperative rehabilitation should include a physical therapy program in which the range of motion of the arm that was achieved in the operating room is not exceeded.(ABSTRACT TRUNCATED AT 250 WORDS)