['Monitoring of end-tidal CO2 levels, performed routinely nowadays in most operating rooms, is obligatory in our hospital for all anesthesia patients. Levels are dependent on pulmonary blood flow, ventilation, and CO2 content of blood. When ventilation is kept constant, the end-tidal CO2 closely follows pulmonary blood flow.', 'Reduction of end-tidal CO2 in the expired air was used to adjust tightness of the pulmonary band in 10 patients with complex cardiac anomalies, all including ventricular septal defect, who underwent pulmonary artery banding. Other parameters were systemic blood pressures and distal pulmonary artery pressures.', 'There were no operative deaths. Average reduction was 3.8 mm Hg (range, 2 to 10 mm Hg; p < 0.001 by paired t test), average increase in systolic blood pressure was 14 mm Hg (range, 4 to 20 mm Hg; p < 0.03 by Wilcoxon sign rank test), distal pulmonary artery pressure was reduced from 56 mm Hg (range, 37 to 79 mm Hg) to 29 mm Hg (range, 20 to 38 mm Hg; p < 0.03 by t test), and postoperative pulmonary artery to systemic pressure ratio averaged 0.36 mm Hg (range, 0.24 to 0.49 mm Hg, difference from preoperative value, p < 0.06).'] Are end-tidal CO2 levels a reliable indicator of band tightness in pulmonary artery banding?