During the past twenty-five years a rather uncommon complication of mitral stenosis, paralysis of the left recurrent laryngeal nerve, has caused considerable interest. The chief importance of the condition lies, perhaps, in the diagnosis, more especially in its distinction from aortic aneurysm; but in addition there has been a great deal of controversy and discussion as to the method of production of the paralysis and its course and termination. Since Ortner first described the condition (1897), four different theories and many modifications have been proposed as to its mechanism, and each has for its support not only the opinion of various clinical investigators, but also the findings in a number of necropsies. Seventy-four cases have been reported, in seventeen of which necropsy was performed. It is my purpose to report a case diagnosed, treated and examined postmortem in the Montreal General Hospital; to review the literature; to discuss the diagnosis,