【Background】Exercise-related variables, heart rate response (dHR), peak oxygen uptake (pVO₂), and ventilatory efficiency (pVE/VCO₂), predict morbidity and mortality in adult patients with congenital heart disease (CHD). However, these predictive values remain unclear in pediatric patients.【Aim】To clarify the prognostic value of exercise-related variables in pediatric CHD patients with or without definitive repair.【Methods】A total of 1,010 cyanotic CHD patients who had undergone exercise testing (CPX) (60 unrepaired, 254 Fontan, 404 tetralogy of Fallot, 162 transposition of the great arteries, others) were reviewed. Exercise-related variables at last CPX in total and adults (n = 390) and those at first CPX in pediatric patients (n = 846) were analyzed. Severity of these variables were graded according to the standardized values; i.e., normal (N) = within ± 2SD, impaired (I) ranged from ± 4SD and ± 2SD, severely impaired (SI) ranged greater ± 4SD. Events included death, arrhythmias, heart failure, re-operation, and catheter intervention which required hospitalization.【Results】Follow-up period (yrs) was 3.3 ± 3.5, 6.2 ± 4.9, 2.7 ± 3.1 for total, pediatric and adult patients, respectively, and 303, 71, and 146 events, including 30, 15, and 45 deaths, were documented, respectively. Free rate (%) from morbidity and mortality at 5, 10, and 15 years after CPX were 82.2, 72.2, and 62.5%, and 94.5, 89.5, and 87.3%, respectively. For all variables, the free rates were higher SI followed by I and N (p < 0.0001) and there was no difference in predictive power for mortality between dHR and pVO₂. In children, pVE/VCO2 had a strong predictive power for morbidity and mortality, while the others did not. In adults, all 3 variables had a significant predictive power of morbidity and mortality.【Conclusion】Our study for the first time demonstrated a significant predictive power of exercise-related variables in pediatric CHD patients and pVE/VCO₂ is a most powerful predictor for future events in pediatric and adult CHD patients.