Open lung biopsy specimens in 72 patients were submitted for evaluation of pulmonary vascular disease. In nine instances, the specimens were inadequate for this purpose. Essentially, there were three indications for taking a lung biopsy specimen. The first was unexplained pulmonary hypertension with 40 patients in this category; plexogenic pulmonary arteriopathy was diagnosed in 14, and chronic pulmonary thromboembolism in 12. Others included pulmonary venoocclusive disease in two patients; two other patients had pulmonary vascular lesions in their lung biopsy specimens, indicative of chronic hypoxic pulmonary hypertension; and ten patients had severe arterial as well as venous alterations that could have been caused by obstruction to pulmonary venous flow as well as by fibrosis of lung tissue. Since we realized this possible source of error, we have submitted these two alternative possibilities to the clinician. In a heterogeneous group of eight patients, the nature of acquired or congenital heart disease remained doubtful in spite of extensive clinical investigation. In several instances, the biopsy specimen contributed to establishing the diagnosis of the cardiac condition. Finally, in a group of 15 patients with known congenital cardiac disease and pulmonary hypertension, whose state of pulmonary vasculature was considered borderline as far as the possibility of corrective surgery of the cardiac defect was concerned, lung biopsy specimens helped to determine the feasibility of surgery. The results show that in a restricted group of patients with pulmonary vascular disease, a biopsy of the lung has a place in establishing the nature and severity of the vascular alterations.