The question as to whether essential or primary pulmonary hypertension actually exists has not been conclusively settled. This condition has been defined by Dresdale and his associates as an elevation of pulmonary artery pressure without demonstrable cause.1 These authors compare primary pulmonary hypertension with essential systemic hypertension and point out that in the latter there is increased tonus of the precapillary arterioles, while in the former there would seem to be a state of increased tonus of the small pulmonary arteries. They also state that the respective vascular beds, peripheral or pulmonary, show similar histologic patterns ranging from normal vessels to obliterative arteriolosclerosis. However, in the great majority of cases reported in the literature as primary pulmonary hypertension, significant vascular changes have been demonstrated in the small arteries of the pulmonary circuit. Dresdale and associates feel that such sclerotic changes, when present, are secondary to the prolonged pulmonary hypertension.