Inappropriate sequencing of activation of shoulder muscles can cause shoulder instability. The records of 868 cases of recurrent shoulder instability referred to a specialist shoulder service between 1981 and 2003 were reviewed. All patients were assessed clinically. Muscle patterning abnormality (Bayley, 1986) was identified in 387 patients (45%). Confirmatory functional electromyography was performed in 97 (25%). Inappropriate pectoralis major activation was identified in cases of anterior instability. In posterior instability, inappropriate activation of latissimus dorsi and anterior deltoid, and suppression of infraspinatus were the classical patterns. Arthroscopic assessment was performed in 179 (46%), identifying additional structural lesions of instability in 96 (54%). All patients diagnosed with muscle patterning disorder received specialist physical therapy using biofeedback. Symptomatic improvement or stability was achieved in 85% of patients with anterior instability and 88% of those with posterior instability. Prior surgical stabilisation procedures on the shoulder reduced cure rates by 5 times for anterior and by 10 times for posterior instability. Muscle patterning abnormalities contribute to recurrent instability of the shoulder in 45% of cases. The success of physical therapy in these patients is high.