Twenty-four children and young adults with electrocardiographic evidence of sinus node dysfunction (SND) are described. The patients, whose ages ranged from three days to 25 years, were divided into three groups based on the type of SND. Ten patients had persistent and inappropriate sinus bradycardia (group I): twelve patients had episodes of prolonged sinus arrest (group II); and seven patients had repeated episodes of Mobitz type I or II sinoatrial exit block (group III). Eighteeen patients had significant heart disease and in 11 of these the SND followed corrective cardiac surgery. Seven were operations for atrial septal defect and transposition repair. Seven patients with SND were symptomatic, having either syncopal (3) or near syncopal episodes (4). All three with syncope have received permanent demand pacemakers. These data show that sinotrial exit block occurs in many young patients with SND (29% in this series). Its recognition, however, requires careful electrocardiographic analysis in order to differentiate sinus bradycardia and sinus arrhythmia. Although SND most commonly occurs in children with congenital or acquired heart disease, particularly following corrective cardiac surgery, it may occur in the absence of other cardiac abnormalities.