Recent studies have shown that arthroscopic lavage of the glenohumeral joint within 10 days following a primary anterior dislocation significantly lowers the recurrence rate when compared with a nonoperative regime. We hypothesize that the lavage reduces distension in the joint and thereby facilitates adaptation and healing of the soft tissue lesion. Using ultrasound, we assessed the hemarthrosis in the glenohumeral joint weekly in 16 consecutive patients after traumatic primary anterior shoulder dislocation. The patients were randomized into two groups for treatment with either arthroscopic lavage or a nonoperative regime. Except for the lavage the two groups followed an identical rehabilitation program. Transversal dorsal ultrasound of the glenohumeral joint was performed, in which the joint effusion was assessed as the distance between the humeral head and the glenoid. Prior to the lavage the two groups had a similar amount of excessive joint effusion. The effusion declined to a steady state level within 3-7 weeks. The joint effusion decreased more rapidly (33%) in the group treated with arthroscopic lavage (P = 0.02) than in the nonoperated group.