A woman, 45 years old at the time of her death, showed the clinical features of primary pulmonary hypertension and the results of cardiac catheterization and pulmonary function tests were consistent with this diagnosis. Histological examination of the lung showed that the pulmonary veins were severely involved by cellular fibrous tissue. These distinctive lesions appear to constitute a separate disease entity for which the name "pulmonary veno-occlusive disease" is suggested.