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Posterior glenohumeral dislocation is less common than anterior dislocation, and less is known about its epidemiology, functional outcome, and complications. The purposes of this study were to determine the epidemiology and demographics of posterior dislocations and to assess the risk of recurrence and the functional outcome after treatment.We performed a retrospective review of a prospective audit of the cases of 112 patients who sustained 120 posterior glenohumeral dislocations. Patients were treated with relocation, immobilization, and then physical therapy. Functional outcome was assessed with the Western Ontario Shoulder Instability Index (WOSI) and the limb-specific Disabilities of the Arm, Shoulder and Hand score (DASH) during the two years after the dislocation.The prevalence of posterior dislocation was 1.1 per 100,000 population per year, with peaks in male patients between twenty and forty-nine years old, and in the elderly patients over seventy years old. Most dislocations (67%) were produced by a traumatic accident, with most of the remainder produced by seizures. Twenty patients (twenty-three shoulders) developed recurrent instability. On survival analysis, 17.7% (95% confidence interval, 10.8% to 24.6%) of the shoulders developed recurrent instability within the first year. On multivariable analysis, an age of less than forty years, dislocation during a seizure, and a large reverse Hill-Sachs lesion (>1.5 cm3) were predictive of recurrent instability. Small persistent functional deficits were detected with the WOSI and DASH at two years.The prevalence of posterior dislocation is low. The most common complication after this injury is recurrent instability, which occurs at an early stage in 17.7% of shoulders within the first year after dislocation. The risk is highest in patients who are less than forty years old, sustain the dislocation during a seizure, and have a large humeral head defect. The risk is lower for most patients who sustain the injury from a traumatic accident, especially if they are older and have a small anterior humeral head defect. There are persistent deficits of shoulder function within the first two years after the injury.Prognostic Level II. See Instructions to Authors for a complete description of levels of evidence.