Arthrography of the shoulder was performed on 50 patients with acute traumatic shoulder dislocations. The mean interval between injury and arthrogram was 2.3 days, with a median interval of one day. Anterior dislocations occurred in 96% of patients, and posterior dislocations in 4%. Initial dislocations were present in 74%, and recurrent dislocations in 26%. The most frequent abnormality identified on arthrograms was an enlarged but intact shoulder capsule (58%), most commonly seen in shoulders subject to recurrent dislocations (77%). Shoulder joint capsular tears or disruptions were seen in 48% of the patients. The next most common lesion was fracture (52%), identified on standard roentgenograms. These included Hill-Sachs lesions (28%), greater tuberosity fractures (22%), and coracoid fractures (2%). Ruptures of the rotator cuff, present in 28% of the cases (14 patients), occurred more frequently in initial dislocations (62%) than in recurrent ones (36%). The high frequency of enlarged intact shoulder capsules, even after an acute initial joint dislocation (58%), suggests that the humeral head does not routinely rupture the capsule during dislocation, but rather tears the glenoid labrum at its bony attachment and dislocates subperiosteally, dissecting a false pouch below the periosteum and under the subscapularis.