Thirty patients with symptomatic sinus node disease (SND) who 5 years previously had undergone a clinical investigation including intracardiac electrophysiologic studies with pharmacologic inhibition of autonomic tone were followed up with respect to development of arrhythmias and cardiac conduction abnormalities. In 17 of these patients a reinvestigation, including a second intracardiac study, was performed. Stable atrial fibrillation developed in five patients, paroxysmal supraventricular tachycardia occurred in one patient, and complete heart block appeared in another patient. Paroxysmal atrial arrhythmias and radiographic atrial enlargement were more common at the initial investigation in patients who during follow-up developed either stable atrial fibrillation or paroxysmal supraventricular tachycardia. Development of complete heart block was associated with signs of severe conduction defects at the initial study. General progression of conduction dysfunction as evaluated by intracardiac techniques was not a finding in this study. Conduction abnormalities limited to the atrioventricular (AV) node did not in this context predict a clinically important progression. The results indicate that those patients with SND who are predisposed to develop high-grade AV block and atrial arrhythmias can be identified by Holter monitoring, chest x-ray examination, and an intracardiac electrophysiologic study. These possibilities improve the selection of patients suitable for pacing modes preserving atrial transport.