Two hundred and fifty-six of 257 shoulders in 254 patients who were between the ages of twelve and forty years and who had primary anterior dislocation of the shoulder that was treated by immobilization for at least three weeks or by early movement were followed for five years in a prospective multicenter study. Two or more recurrences had occurred in 55 per cent of the shoulders in patients who were twenty-two years old or younger, in 37 per cent of the shoulders in patients who were twenty-three to twenty-nine years old, and in 12 per cent of the shoulders in patients who were thirty to forty years old. Surgery for the treatment of instability had been performed or was scheduled to be performed in 28, 18, and 5 per cent of the shoulders, respectively, in the three age-groups. The dislocation recurred in only one of thirty-two shoulders that had a fracture of the greater tuberosity (p less than 0.001). The results as regards recurrence after five years were not influenced by immobilization of the shoulder for three to four weeks after the initial dislocation, bilateral dislocation, or the type of initial trauma and athletic activity. A moderate impression fracture of the humeral head (Hermodsson or Hill-Sachs) did not change the prognosis in the younger patients (twenty-two years old or younger). However, in the patients who were twenty-three to forty years old the rate of recurrence was significantly higher (p less than 0.01) when this injury was demonstrated.(ABSTRACT TRUNCATED AT 250 WORDS)