The difficulties involved in trying to establish the clinical presence of such a common entity as pulmonary embolism, particularly as to its degree and distribution, are such that the clinician can seldom feel secure in his diagnosis and must rely heavily on probability, a factor which is developed to a considerable extent in a given case by the exclusion of other likely causes of such symptoms as chest pain, fever, and tachypnea. The case to be presented represents an extraordinary aspect of this problem, and because of its unusual features it also affords information particularly about the remarkable reserve of the lesser circulation. Report of a Case A 58-year-old Negro male was admitted to the University Hospital on Aug. 12, 1960, because of severe right-sided chest pain and shortness of breath. One month previously he had been seen in the Accident Room of the hospital because of the sudden onset