The scalenus anticus syndrome has occurred with unexpected frequency in members of the armed forces of the United States. Its incidence has been sufficiently great to warrant emphasis on the ease with which the syndrome can be recognized and treated. Furthermore, certain observations of both clinical and physiologic importance should be recorded. Characteristic neurologic signs, consisting of sensory changes, muscular spasm and weakness, were noted among the patients studied. The distribution and nature of these signs, together with their response to appropriate therapy, suggest that they were due to compression of cervical nerve roots. The mechanism by which the lower roots are compressed by the scalenus anticus muscle is fairly well understood, but the means by which the upper roots are compressed needs to be clarified. In an attempt to do this, a detailed anatomic account of the scalene muscles and their pertinent relations will be presented. For a review