Among 80 cases with a clinical diagnosis of primary (unexplained) pulmonary hypertension, 45 (56%) had thromboembolic disease and 22 (28%) had plexogenic arteriopathy; the remaining 13 (16%) had pulmonary veno-occlusive disease, primary medial hypertrophy, primary pulmonary arteritis, or changes consistent with pulmonary venous hypertension. The mean age was 16 years for primary pulmonary arteritis, 21 to 34 years for plexogenic pulmonary arteriopathy, primary medial hypertrophy, and pulmonary veno-occlusive disease, and 41 and 45 years for thromboembolic disease and pulmonary venous hypertension, respectively. In all forms except pulmonary veno-occlusive disease and apparent pulmonary venous hypertension, female patients were involved twice as often as male patients. With the exception of apparent pulmonary venous hypertension, patients with plexogenic pulmonary arteriopathy had the longest survival (63 months). Sudden death, however, occurred most frequently in patients with plexogenic disease (45%) and occurred 2.5 times as often in this group as in patients with thromboembolic disease. Among our 80 cases, the most frequent histopathologic lesions were medial hypertrophy, intimal proliferation and fibrosis, fibrinoid degeneration and necrosis, and thrombosis. Thrombi were commonly observed and may have developed in situ or by embolization; they were often rich in platelets when they occurred in small pulmonary vessels. These histologic features may form the morphologic substrate for elevated pulmonary vascular resistance, and their recognition may provide the rationale for possible intervention with pulmonary vasodilators, anticoagulants, or platelet inhibitors.