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This study was undertaken to detect diagnostic ultrastructural changes in the alveolar walls in biopsies of patients with primary pulmonary hypertension (PPH). Open lung biopsies from 8 patients referred to the Hospital of the University of Pennsylvania were available for study. By light microscopy 4 patients were diagnosed as having plexogenic pulmonary arteriopathy, 2 thromboembolic arteriopathy and 2 veno-occlusive (PVOD) disease. In 4 patients the diagnosis of PPH was further confirmed at autopsy. Changes were observed in the basement membrane of the alveolar capillaries in 6 of the patients. These consisted in marked thickening and duplication of the basement membrane of alveolar capillaries in the 4 patients with the plexogenic form of pulmonary hypertension. Patients with PVOD instead showed uniform thickening of the basement membrane. No changes were detected in the 2 biopsies of patients with the thromboembolic form of PPH. We conclude that ultrastructural studies contribute significantly to the precise characterization of pathologic changes in the lungs of patients with PPH.