No condition is less often diagnosed correctly than massive pulmonary embolism. The purpose of this communication is to show that (1) a high percentage of diagnotic errors is made in it, (2) that there are definite reasons for these mistakes, and (3) that there appears to be a possibility of increasing general diagnostic accuracy by systematic examination of the patient for certain physical signs. Until Herrick's1classical description of acute myocardial infarction in 1912, sudden deaths were frequently and often erroneously attributed to massive pulmonary embolism. In recent years the pendulum has swung in the other direction. Sudden deaths occurring at the present time are prone to be ascribed to acute myocardial infarction. It is true that in any large series of sudden deaths, as Hamman2has pointed out, 40% will be found to have myocardial infarcts, and only 5% pulmonary emboli. The odds, therefore, are 8 to