A brief historical review of primary pulmonary arteriosclerosis and hypertension has been given. Ten additional cases with detailed laboratory, roentgenographic, electrocardiographic, cardiac catheterization, and angiocardiographic studies are presented. The findings on 4 necropsied cases are also included. Progressive exertional shortness of breath, syncope, left chest pain, right ventricular hypertrophy, and pulmonary arterial dilatation, combined with high right ventricular and pulmonary arterial pressure and normal pulmonary capillary pressure in the absence of pulmonary disease, should be extremely suggestive of primary pulmonary hypertension. An unrelenting downhill course of right ventricular failure is usually seen.