Abstract Objectives To evaluate the long-term clinical outcome and quality of life up to 48 years of patients with Transposition of the Great Arteries (TGA) after Mustard procedure at young age. Primary outcome was defined as a composite endpoint of death, re-intervention, clinically relevant heart failure or arrhythmia, ischemic stroke and endocarditis. Methods Single center, longitudinal cohort study evaluating consecutive patients with TGA who underwent Mustard procedure before the age of 15 between 1973 and 1980 at our medical center, the Netherlands. Survival of all eligible patients was obtained. All eligible patients were invited for in hospital evaluation and underwent extensive cardiac and psychological evaluation every 10 years. The latest evaluation took place in 2021 and 2022. Results The original study cohort consisted of 91 consecutive patients. Cumulative survival at 48 years (95% complete) was 62% and significantly decreased compared to the general Dutch population (p<0.001). Cumulative survival at 48 years in hospital survivors was 66%. Mortality rate was 0.19 per 100 patient-years. Thirty-four patients (77%) of the eligible 44 survivors (71% male, median age at study 46 years) were included in the present study. Median follow-up since initial surgical correction was 46 (range 40-48) years. Event-free survival at 48 years was 14%. Overall, 34 patients underwent re-intervention of which 3 in the last 10 years (predominantly Baffle revision). In total, 29 patients had supraventricular tachycardia and 11 patients had ventricular tachycardia of which 8 and 4 respectively occurred in the last 10 years. Alltogether, 22 patients had symptomatic heart failure of which 9 occurred in the last 10 years. Cardiac magnetic resonance imaging (n=11) showed a median systemic right ventricular ejection fraction (sRVEF) of 35% [32-40%]. In 73% of the patients, sRVEF fraction was below 40%. Echocardiography showed a diminished sRVEF in all patients. VO2max was reduced in 95% of all patients. Self-perceived quality of life (SF-36) generally was comparable to the general Dutch population with the exception of domain general health (p<0.001). Conclusion Late mortality, and especially morbidity of hospital survivors after Mustard procedure was substantial, with an event free survival of only 14% at 48 years. Re-intervention and symptomatic arrhythmia were often reported. The function of the systemic right ventricle was diminished and VO2max was also reduced. The burden of heart failure increased drastically in the last decade. Therefore, intensive follow-up of these patients is warranted.