Owing to the exceptional mechanical precariousness of the glenohumeral joint, dislocations and instabilities are common. Because posterior dislocation or instability of the joint occurs infrequently, these abnormalities may be misdiagnosed on routine radiographic examinations in acute shoulder injuries. We present five patients in whom sonographic examination was used to diagnose posterior shoulder instability (three cases) and posterior shoulder dislocation (two cases). The sonographic technique demonstrates the presence of instability or dislocation and permits dynamic examination of the shoulder joint. Ultrasonography emerges as a useful and efficient novel technique for detecting these disorders. The accuracy, sensitivity, and specificity of the technique presented here are currently being determined.