SUMMARY Pulmonary embolic disease is the most serious disturbance of the lungs, often striking without warning and causing death within a few minutes when least expected. The clinician should consider the possibility that pulmonary embolism is present in many instances of complications referable to the lungs, particularly among patients more than 40 years of age, patients who have sustained injury or undergone an operation, those bedridden from any cause, those who have chronic disease, especially heart disease and malignant conditions, but particularly among those with thrombophlebitis or a history of it and those who previously have had pulmonary embolism. Even after using every means at his disposal, the clinician still may have difficulty in making the diagnosis correctly, and the over-all accuracy of diagnosis in cases in which the outcome ultimately proves fatal is perhaps no better than 50 per cent, an arresting fact. Death may occur early, but treatment may often prove lifesaving. Therefore, early accurate diagnosis and early treatment are imperative. New methods of diagnosis and treatment present greater hope for the future.