Previous radionuclide studies have shown residual maldistribution of lung perfusion after intracardiac repair of Tetralogy of Fallot (TF). Maldistribution of perfusion may also be detectable by measurements of gas exchange during exercise. Thus, we used exercise to test for maldistribution of perfusion in 13 children (8 to 18 yr of age) who were clinically well (New York Heart Association Class 1) 7 to 14 yr after repair of TF. Sixteen children, matched to the study group by age, size, and sex, served as control subjects. Peak oxygen consumption during progressive exercise on a cycle ergometer was 28.7 +/- 6.6 (SD) ml/kg/min for the study group compared with 35.7 +/- 6.9 for the control subjects (p less than 0.05). During steady-state exercise at a VCO2 of 0.6 L/min, ventilation (VE) and the ventilatory equivalents for oxygen (VE/VO2) and carbon dioxide (VE/VCO2) were high (VE/VCO2 = 35.9 +/- 4.4 versus 32.0 +/- 3.5, p less than 0.05), whereas mixed expired and end-tidal CO2 concentrations were low (PETCO2 = 34.0 +/- 2.4 versus 39.2 +/- 3.0 mm Hg, p less than 0.001). Indices of pulmonary function were normal; FVC values were 96 +/- 17% and FEV, values were 96 +/- 16% of predicted values. Therefore, children who are clinically well may exhibit gas exchange abnormalities compatible with mild maldistribution of lung perfusion 7 to 14 yr after repair of TF.