OBJECTIVE To evaluate the prevalence and prognostic significance of asymptomatic complex or frequent ventricular premature beats detected during ambulatory electrocardiographic (ECG) monitoring. METHODS Cohort study with a follow-up period of 4 to 6 years. METHODS Population-based. METHODS Surviving patients of the original Framingham Heart Study cohort and offspring of original cohort members (2727 men and 3306 women). METHODS One-hour ambulatory electrocardiography. RESULTS The age-adjusted prevalence of complex or frequent arrhythmia (more than 30 ventricular premature complexes per hour or multiform premature complexes, ventricular couplets, ventricular tachycardia, or R-on-T ventricular premature complexes) was 12% (95% Cl, 11% to 13%) in the 2425 men without clinically evident coronary heart disease and 33% (Cl, 24% to 42%) in the 302 men with coronary heart disease. The corresponding values in women (3064 without disease and 242 with disease) were 12% (Cl, 11% to 13%) and 26% (Cl, 9% to 43%). After adjusting for age and traditional risk factors for coronary heart disease in a Cox proportional hazards model, men without coronary heart disease who had complex or frequent ventricular arrhythmias were at increased risk for both all-cause mortality (relative risk, 2.30; Cl, 1.65 to 3.20) and the occurrence of myocardial infarction or death from coronary heart disease (relative risk, 2.12; Cl, 1.33 to 3.38). In men with coronary heart disease and in women with and without coronary heart disease, complex or frequent arrhythmias were not associated with an increased risk for either outcome. CONCLUSIONS In men who do not have clinically apparent coronary heart disease, the incidental detection of ventricular arrhythmias is associated with a twofold increase in the risk for all-cause mortality and myocardial infarction or death due to coronary heart disease. The preventive and therapeutic implications of these findings await further investigation.