Background: The etiopathogenesis of electrocardiographic bundle branch and atrioventricular blocks is not fully understood. We investigated familial clustering of cardiac conduction defects and pacemaker insertion in the FHS (Framingham Heart Study). Additionally, we assessed familial clustering of pacemaker insertion in the Danish general population. Methods: In FHS, we used multivariable-adjusted logistic regression models to investigate the association of parental atrioventricular block (PR interval, ≥0.2 s), complete bundle branch block (QRS, ≥0.12 s), or pacemaker insertion with the occurrence of cardiac conduction abnormalities in their offspring. The Danish nationwide administrative registries were interrogated to assess the relations of parental pacemaker insertion with offspring pacemaker insertion. Results: In FHS (n=371 cases with first-degree atrioventricular block, complete bundle branch block, or pacemaker insertion, and 1471 age- and sex-matched controls), individuals with at least 1 affected parent with a conduction defect had a 1.65-fold odds (odds ratio, 95% CI, 1.32-2.07) for manifesting an atrioventricular block and a 1.62-fold odds (95% CI, 1.08-2.42) for developing a complete bundle branch block. If at least 1 parent had any electrocardiographic conduction defect or pacemaker insertion, the offspring had a 1.62-fold odds (95% CI, 1.31-2.00) for experiencing any of these conditions. In Denmark (n=2 824 199 individuals; 5397 incident pacemaker implantations), individuals with at least 1 first-degree relative with history of pacemaker insertion had a multivariable-adjusted 1.68-fold (incidence rate ratio, 95% CI, 1.49-1.89) risk of undergoing a pacemaker insertion. If the affected relative was ≤45 years of age, the incidence rate ratio was markedly increased to 51.0 (95% CI, 32.7-79.9). Conclusions: Cardiac conduction blocks and risk for pacemaker insertion cluster within families. A family history of conduction system disturbance or pacemaker insertion should trigger increased awareness of a similar propensity in other family members, especially so when the conduction system disease occurs at a younger age.