We undertook a prospective study using arthroscopy to determine the intraarticular derangement caused by initial anterior shoulder dislocations. Of our 32 patients, 15 (47%) had a grade I, grade II, or grade III Hill-Sachs lesion. The patients were 29 men and 3 women whose ages ranged from 15 to 28 years (mean 22 years). All of the patients studied were United States military personnel or members of their families. None of the patients had had prior injuries of the affected shoulder, and all underwent arthroscopy within 72 h of injury. Few reports describe a Hill-Sachs lesion in a patient following a single anterior dislocation. Early reports of this lesion were based on indirect (radiographic) evidence. Our arthroscopic findings indicate that the incidence of this lesion after one anterior dislocation is higher than previously thought.