Posterior dislocation of the humerus is a diagnostic trap. Clinical and roentgenographic evidence of this lesion is always present, but usually escapes notice unless educed. Recent lesions, recognized and treated at once, respond rapidly and well to simple measures. Recurrent or unreduced lesions require operative treatment. Irreparable damage to the glenohumeral articulation does not always require arthrodesis. The most common actual and serious potential complication of posterior dislocation of the humerus is the failure to arrive at the correct diagnosis prior to commencing treatment. Some failures of surgical treatment for habitual anterior dislocation of the humeral head result from the fact that the recurrent episodes were in reality posterior subluxations of the humerus.