For evaluation of pulmonary vascular disease 140 open lung biopsies were performed in 137 patients with congenital heart disease in order to decide whether the state of the lung vessels would allow corrective surgery. As far as possible follow-up was obtained in patients who underwent a cardiac repair. From the study of these biopsy specimens it appeared that medial hypertrophy of pulmonary arteries and changes in pulmonary veins should not be considered an impediment for a corrective operation as long as more advanced changes are absent. The same is true for intimal thickening due to longitudinal smooth muscle, post-thrombotic changes or cellular proliferation. Concentric-laminar intimal fibrosis forms no contra-indication as long as it is mild but, if severe, it is likely that hypertensive pulmonary vascular disease will progress in spite of cardiac repair. In the presence of fibrinoid necrosis or plexiform lesions correction of a cardiac anomaly should not be attempted. The presence of dilatation lesions is more disputable but probably, as long as they are scarce and not accompanied by fibrinoid necrosis or plexiform lesions, corrective surgery may be attempted. This becomes very dubious when they are more numerous.