Pulmonary embolism in medical patients is not infrequent and, contrary to general opinion, is actually commoner than pulmonary embolism occurring postoperatively. It has often been overlooked during the life of the patient even by experienced physicians, and it is still clinically diagnosed in probably less than half of the medical patients in whom it is discovered at autopsy. It was the experience of one of us (P. D. W.)1five years ago that pulmonary embolism was diagnosed personally nearly ten times as often in the decade from 1931 to 1940 inclusive as in the previous decade from 1921 to 1930, although the patients suffered from the same kinds and degrees of cardiac disease in the two decades. The answer obviously was twofold: first, the possibility of this condition was borne in mind and second, the criteria for diagnoses became more clearly defined with experience. Death from pulmonary embolism is