Primary pulmonary hypertension is a distinct clinicopathologic entity characterized by pulmonary arterial and right ventricular hypertension. Right ventricular hypertrophy is uniformly present and may or may not be accompanied by right ventricular failure. The hypertension is the result, hemodynamically, of excessive increase in pulmonary vascular resistance and may reflect itself in the pulmonary vessels by a variety of vascular lesions. The pathogenesis of the ultimate vascular lesions remains obscure. The clinical, hemodynamic, and pathologic features of four patients with this condition have been presented. The clinical findings of cases reported in the literature as well as in our cases show a remarkable uniformity which, in our interpretation, is a reflection of the pulmonary hypertension. Thus, it is difficult if not impossible on clinical grounds alone to differentiate this lesion from any of the multitude of conditions in which pulmonary hypertension is a secondary concomitant. Cardiac catheterization has provided valuable information on the pathophysiology of primary pulmonary hypertension and may be of great value in differential diagnosis, particularly in excluding congenital heart disease. The abnormal hemodynamic data that may be demonstrated by this technic, however, are not sufficiently specific to permit one to make a conclusive diagnosis. In our opinion, such a definitive diagnosis is possible only by necropsy, and even here the exact mechanism may be difficult to define, although it is apparent now that many of the lesions in the pulmonary arteries represent the effect of pulmonary hypertension. In the face of such diagnostic difficulties and because there is no satisfactory treatment for primary pulmonary hypertension, it is in the patient's best interest that a search for a lesion that can be treated should not be abandoned.