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Background: The prevalence and risk factors for recurrent instability and functional impairment following a primary glenohumeral dislocation remain poorly defined in younger patients. We performed a prospective cohort study to evaluate these outcomes. We also aimed to produce guidelines for the design of future clinical trials, assessing the efficacy of interventions designed to improve the outcome after a primary dislocation.Methods: We performed a prospective cohort study of 252 patients ranging from fifteen to thirty-five years old who sustained an anterior glenohumeral dislocation and were treated with sling immobilization, followed by a physical therapy program. Patients received regular clinical follow-up to assess whether recurrent instability had developed. Functional assessments were made and were compared for two subgroups: those who had not had instability develop and those who had received operative stabilization to treat recurrent instability.Results: On survival analysis, instability developed in 55.7% of the shoulders within the first two years after the primary dislocation and increased to 66.8% by the fifth year. The younger male patients were most at risk of instability, and 86.7% of all of the patients known to have recurrent instability had this complication develop within the first two years. A small but measurable degree of functional impairment was present at two years after the initial dislocation in most patients. Sample-size calculations revealed that a relatively small number of patients with a primary dislocation would be required in future clinical trials examining the effects of interventions designed to reduce the prevalence of recurrent instability and improve the functional outcome.Conclusions: Recurrent instability and deficits of shoulder function are common after primary nonoperative treatment of an anterior shoulder dislocation. There is substantial variation in the risk of instability, with younger males having the highest risk and females having a much lower risk. Future clinical trials to evaluate primary interventions should evaluate the prevalence of recurrent instability and functional deficits, with use of an assessment tool specifically for shoulder instability, during the first two years after the initial dislocation.Level of Evidence: Prognostic Level I. See Instructions to Authors for a complete description of levels of evidence.