BACKGROUND Serum gamma-glutamyl transferase activity (GGT) seems to predict cardiovascular events in different populations. However, no data exist on patients with congenital heart disease (CHD). METHODS Observational, analytic, prospective cohort study design involving CHD patients and a control population to determine the effect of GGT levels on survival. RESULTS A total of 589 CHD patients (58% males, 29 ± 14 years old) and 2745 matched control patients were followed up. A total of 69 (12%) CHD patients had a major acute cardiovascular event (MACE) during the follow-up time (6.1 [0.7-10.4] years). Patients with CHD and a GGT >60 U/L were significantly older, more hypertensive and dyslipidemic, had a worse NYHA functional class and a greater anatomical complexity than CHD patients with a GGT ≤60 U/L. The binary logistic regression analysis showed that age, a great CHD anatomical complexity, and having atrial fibrillation/flutter were the predictive factors of higher GGT levels (>60 U/L). The Kaplan-Meier analysis showed that patients with CHD and a GGT concentration above 60 UL showed the lowest probability of survival compared to that of CHD with GGT ≤60 U/L and controls irrespective of their GGT concentrations (p < .001). Similarly, the multivariable Cox regression analysis found an independent association between higher GGT levels (>60 U/L) and a worse prognosis (HR 2.44 [1.34-4.44], p = .003) among patients with CHD. CONCLUSIONS Patients with CHD showed significant higher GGT levels than patients in the control group having those with higher GGT concentrations (>60 U/L) the worst survival.