A retrospective review was made of 59 open lung biopsy specimens taken between 1984 and 1988 from children with congenital heart disease who were at risk for pulmonary vascular disease. Thirty-seven patients (ranging in age from 3.5 months to 23 years; median age, 14 months) had a primary left-to-right shunt (group A) and 22 patients (ages 1 to 15 years) had palliated cyanotic heart disease (group B). Forty-five of the lung biopsy specimens were requested as frozen sections. In both groups lung biopsy specimens were graded by the Heath-Edwards classification and correlated against preoperative hemodynamic data and outcome. In group A patients, carefully measured pulmonary vascular resistance and pulmonary/systemic vascular resistance ratio were reliable indicators of the structural state of the pulmonary vascular bed, obviating the need for routine lung biopsy. Pulmonary/systemic vascular resistance ratios greater than 0.45 accurately predicted all patients with irreversible pulmonary vascular disease, and pulmonary vascular resistance greater than 7 units.m2 accurately predicted all but one case of disease. Reversibility of pulmonary vascular changes is not synonymous with immediate postoperative survival: Fatal postoperative pulmonary hypertensive crises occurred in the presence of reversible pulmonary disease. Of those considered for the Fontan procedure, a mean pulmonary artery pressure less than 30 mm Hg and pulmonary vascular resistance less than 3 units.m2 correlated with Heath-Edwards grade I or normal lung biopsy results. In 36% of group B patients, reliable assessment of pulmonary vascular resistance could not be made, indicating a possible need for open lung biopsy procedures. When lung biopsy procedures were used as an isolated procedure, they were more dangerous (20% mortality, 13% morbidity) than previously reported. Intraoperative frozen sections are not adequate to accurately assess pulmonary vascular changes (9% error); serial paraffin sections are required.