In 78 patients with primary plexogenic arteriopathy (PPA), numbers of organized and recanalized thrombi were established in histologic slides of lung tissue and expressed per square centimeter of section. Three control groups of ten individuals each were used: normal, plexogenic arteriopathy secondary to ventricular septal defect, and hypoxic pulmonary hypertension. Thrombotic lesions were scarce in normal individuals but numerous in all three groups with pulmonary hypertension. There is also a positive correlation with age. Thrombotic lesions are absent or scarce in children but more common in adults, even in normal control subjects and particularly in pulmonary hypertension by whatever cause. In PPA there is likely to be a relation with the duration of illness but not with the stage of the disease. The complete pattern of plexogenic arteriopathy may develop in the absence of thrombotic lesions, which clearly are not essential for its pathogenesis. Rather than being part specifically of PPA, as sometimes suggested, thrombotic lesions complicate various types of hypertensive pulmonary vascular disease. Apparently the combination of sustained pulmonary hypertension and age, possibly through endothelial injury, may elicit thrombosis and its sequelae, which in turn may aggravate the pulmonary arterial pressure.