Description: Background— Systematic follow-up is currently not recommended for patients with simple congenital heart disease; however, only a few data exist on the long-term prognosis of simple congenital heart disease. Methods and Results— We undertook a nationwide follow-up study of a cohort of 1241 simple congenital heart disease patients, diagnosed from 1963 through 1973, in otherwise healthy children and alive at 15 years of age. We identified 10 age- and sex-matched general population controls per patient. We followed the study population through Danish public registries from the age of 15 years up to January 1, 2013 with respect to mortality, cause of death, morbidity, and medical follow-up. The patients were followed for a total of 58 422 patient-years and had a median age at the end of follow-up of 47.4 years (interquartile range, 43.5–50.9). Mortality was increased compared with the general population, both overall (adjusted hazard ratio [aHR],1.9; 95% confidence interval [CI], 1.5–2.4)] and for patients (79%) without medical follow-up (aHR, 1.7; 95% CI, 1.3–2.2). The most common cause of death (40%) was sudden unexpected death (aHR, 4.3; 95% CI, 2.9–6.5). The incidence of critical cardiac morbidity was 3.9 per 1000 patient-years with the most frequent events being an adult (re)operation and hospitalization for heart failure or ventricular tachyarrhythmia. This corresponded to an aHR of 5.7 (95% CI, 4.6–6.9) when compared with the general population. Conclusions— Patients diagnosed with simple congenital heart disease in the 1960s have substantially increased long-term mortality and cardiac morbidity compared with the general population. Further studies on the effectiveness of systematic medical follow-up programs appear warranted.