We describe pathological abnormalities in a 72‐year‐old male member of a family with a congenital absence of sinus rhythm and a tendency to develop atrial fibrillation at an early age, and in a 54‐year‐old female member of a family with cardiomyopathy and progressive conduction system disease manifested by first‐degree atrioventricular (AV) block, left bundle branch block, and atrial arrhythmias. Both patients died suddenly. The absence of sinus rhythm in case 1 could be explained by marked atrophy, degeneration, and isolation of the sinoatrial (SA) node. The SA node was also diseased in the member of the other family with atrial arrhythmias. Additional common features in both cases included: fatty metamorphosis and degenerative changes of the approaches to the SA node, the atrial preferential fibers, and the approaches to the AV node, a small AV node, degenerative changes of the bundle branches, and floppy AV valves. These findings show that the pathological substrate of familial supraventricular arrhythmias consists of a diffuse involvement of the entire conduction system, bearing resemblance to pathological findings in elderly subjects with acquired sick sinus syndrome.