BACKGROUND: Anterior dislocation of the glenohumeral joint in younger patients is associated with a high risk of recurrence and persistent functional deficits. The aim of this study was to assess the efficacy of a primary arthroscopic Bankart repair, while controlling for the therapeutic effects produced by the arthroscopic intervention and joint lavage. METHODS: In a single-center, double-blind clinical trial, eighty-eight adult patients under thirty-five years of age who had sustained a primary anterior glenohumeral dislocation were randomized to receive either an arthroscopic examination and joint lavage alone or together with an anatomic repair of the Bankart lesion. Assessment of the rate of recurrent instability, functional outcome (with use of three scores), range of movement, patient satisfaction, direct health-service costs, and treatment complications was completed for eighty-four of these patients (forty-two in each group) during the subsequent two years. RESULTS: In the two years after the primary dislocation, the risk of a further dislocation was reduced by 76% and the risk of all recurrent instability was reduced by 82% in the Bankart repair group compared with the group that had arthroscopy and lavage alone. The functional scores were also better (p < 0.05), the treatment costs were lower (p = 0.012), and patient satisfaction was higher (p < 0.001) after arthroscopic repair. The improved functional outcome appeared to be mediated through the prevention of instability since the functional outcome in patients with stable shoulders was similar, irrespective of the initial treatment allocation. The patients who had a Bankart repair and played contact sports were also more likely to have returned to their sport at two years (relative risk = 3.4, p = 0.007). CONCLUSIONS: Following a first-time anterior dislocation of the shoulder, there is a marked treatment benefit from primary arthroscopic repair of a Bankart lesion, which is distinct from the so-called background therapeutic effect of the arthroscopic examination and lavage of the joint. However, primary repair does not appear to confer a functional benefit to patients with a stable shoulder at two years after the dislocation.