Sixty-nine patients (age 10 +/- 3.5 years, median 9.7 years) operated on for tetralogy of Fallot, underwent exercise testing 5.1 +/- 2.5 years after total correction. Aerobic capacity was assessed by the ventilatory threshold method. After the exercise test, parents completed a validated standardized questionnaire to evaluate the habitual level of physical activity of their child. They were then asked to place their child into one of three groups, describing their child's activity level, compared to normals (below average, average and above average). In the average and above-average groups, 73% and 65% of the patients were misclassified when compared to objective exercise testing. However, in the below-average group, no misclassifications were found. Subjective estimates of activity level, and consequently exercise tolerance, were poorly associated with objective measurements of exercise performance in these patient groups. Our results question the reliability of self-reported estimates of physical activity level and consequently exercise capacity in children after surgical correction of tetralogy of Fallot.