Accessory atrioventricular pathways, the anatomical structures responsible for the preexcitation syndromes, may result from a developmental failure to eradicate the remnants of the atrioventricular connections present during cardiogenesis. To study whether preexcitation syndromes could also be transmitted genetically, we determined the prevalence of preexcitation in the first-degree relatives of 383 of 456 consecutive patients (84 percent) with electrophysiologically proved accessory pathways. We compared the observed prevalence of preexcitation among the 2343 first-degree relatives with the frequency reported in the general population (0.15 percent). For 13 of the 383 index patients (3.4 percent), accessory pathways were documented in one or more first-degree relatives. At least 13 of the 2343 relatives identified (0.55 percent) had preexcitation; this prevalence was significantly higher than that in the general population (P less than 0.0001). Identification of affected relatives may have been incomplete because clinical information was obtained only about symptomatic relatives. Patients with familial preexcitation have a higher incidence of multiple accessory pathways and possibly an increased risk of sudden cardiac death. Our data suggest a hereditary contribution to the development of accessory pathways in humans. The pattern of inheritance appears to be autosomal dominant.