Thirty children and young adults with electrocardiographic and clinical evidence of sinus node dysfunction (SND) are described. Ages ranged from 3 days to 25 years. Of these, 10 patients had inappropriate sinus bradycardia, 16 had periods of sinus arrest, and 9 had sinoatrial exit block. A total of 22 patients had significant heart disease and SND developed in 13 after cardiac surgery. Eight patients with SND had no obvious heart disease. Seven patients were symptomatic, having either syncopal (three) or near-syncopal episodes (four). The three with syncope have received permanent demand pacemakers. Although significant SND occurs most frequently in children with congenital heart disease, particularly following corrective cardiac surgery, it may occur in otherwise normal individuals.