Background - The majority of available data on the clinical course of patients with ventricular preexcitation in the ECG originates from tertiary centers. We aimed to investigate long-term outcomes in individuals from a primary care population with electrocardiographic preexcitation. Methods and Results - Digital ECGs from 328 638 primary care patients were collected during 2001 to 2011. We identified 310 individuals with preexcitation (age range, 8-85 years). Data on medication, comorbidity, and outcomes were collected from Danish nationwide registries. The median follow-up time was 7.4 years (quartiles, 4.6-10.3 years). Compared with the remainder of the population, patients with preexcitation had higher adjusted hazards of atrial fibrillation (hazard ratio [HR], 3.12; 95% confidence interval [CI], 2.07-4.70) and heart failure (HR, 2.11; 95% CI, 1.27-3.50). Subgroup analysis on accessory pathway location revealed a higher adjusted hazard of heart failure for a right anteroseptal accessory pathway (HR, 5.88; 95% CI, 2.63-13.1). There was no evidence of a higher hazard of death among individuals with preexcitation when looking across all age groups (HR, 1.07; 95% CI, 0.68-1.68). However, a statistically significant (P=0.01) interaction analysis (<65 versus ≥65 years) indicated a higher hazard of death for patients with preexcitation ≥65 years (HR, 1.85; 95% CI, 1.07-3.18). Conclusions - In this large ECG study, individuals with preexcitation had higher hazards of atrial fibrillation and heart failure. The higher hazard of heart failure seemed to be driven by a right anteroseptal accessory pathway. Among elderly people, we found a statistically significant association between preexcitation and a higher hazard of death.