Pathologic obstruction of the proximal lumen and secondary atrophy of the media of the peripheral small pulmonary arteries were absolute operative contraindications in cases of VSD and/or PDA with severe pulmonary hypertension. Such patients who were operated on died with no decrease in pulmonary arterial pressure. The index of pulmonary vascular disease (IPVD), a composite and quantitative evaluation of the severity of pulmonary vascular disease, was introduced to determine the operability of other patients. An IPVD rating of 2.2 in Down's syndrome and 2.1 without the syndrome were regarded as the upper permissible limits for surgical intervention based on results of 23 autopsies and 26 lung biopsies of patients operated on before 1981. Open lung biopsy was performed in 51 patients to determine applicability of our operative indications. Twenty-nine cases were considered operable by our criteria, and 28 underwent surgical correction without operative or late death. Twenty-two cases thought inoperable remain under observation. Comparative analysis of the pathology and preoperative hemodynamic data suggested that lung biopsy should be carried out to determine operability in cases with pulmonary vascular resistance greater than 8 units.m2.