Sick sinus syndrome (SSS) occurs infrequently in children who have not undergone cardiac surgery. The symptoms, electrocardiograms, and electrophysiologic data in 11 patients aged 2 to 17 years who had nonsurgical SSS were reviewed. Syncope occurred in 5 patients and sinus bradycardia in 9. Sinus nodal recovery times were prolonged in 6 patients. The atrial effective refractory period was prolonged in 2 patients and the atrioventricular nodal functional or the effective refractory period, or both, was prolonged in 5 patients. Because patients with nonsurgical SSS may have abnormalities not only of the sinus node but also of the atrium and the atrioventricular node, it is recommended that patients with symptomatic SSS be evaluated by electrophysiologic study. The proper choice of antiarrhythmic drug therapy or permanent pacing procedure depends on a complete analysis of the cardiac conduction system.