Essential Pulmonary Hypertension may be defined as an elevation of pulmonary artery pressure without demonstrable cause.This discussion concerns the existence of such an abnormal condition confined to the pulmonary arterial circulation and comparable to essential hypertension of the systemic arterial circulation. Only pulmonary arterial hypertension without co-existing systemic arterial hypertension will be considered. All other forms of cor pulmonale were excluded such as pulmonary fibrosis from all causes, pulmonary emphysema, mitral stenosis, left-sided heart failure from any eticlogy, particularly multiple peripheral organized emboli within the lung, and congenital heart disease such as anomalous pulmonary venous return, septal defects, patent ductus arteriosus.The divergency of opinion as to the existence of primary pulmonary hypertension is well illustrated by the varying opinions recently expressed by the following outstanding authorities in personal communications to the authors: Cournandi wrote, "I cannot deny that the syndrome might exist in very rare instances. It is essential, therefore, to maintain a critical viewpoint concerning this subject and to exhaust all mechanical factors before invoking derangement of a vasomotor control which appears very unlikely as a source of hypertension.' Sprague2 is even more dubious, stating, " I do not believe we recognize a condition'essential pulmonary hypertension' except 133