BACKGROUND: The presence of a single left superior vena cava in the absence of complex congenital heart disease is uncommon, and, in the absence of hemodynamic consequences, it would not be expected to result in cardiovascular signs or symptoms. Single case reports and our anecdotal experience suggested to us that this anomaly is highly associated with cardiac arrhythmias. OBJECTIVE: We sought to describe the clinically important arrhythmias in a population of young patients having this anomaly. METHODS: A retrospective chart review was performed from all patients <20 years old and who were determined by echocardiography over an 11-year-period to have a single left superior vena cava and minor or no coexisting congenital heart defects. The prevalence of nonsinus pacemaker, age-corrected sinus rate percentile, and prevalence of brady- or tachyarrhythmias was compared with a control group of patients having bilateral superior vena cavae. RESULTS: Eight patients having a single left and 55 patients having bilateral superior vena cava(e) were identified. The existence of this anomaly tended to be associated with a lower age-corrected sinus rate percentile (17.5% vs 75%, P = 0.09), and was associated with a higher prevalence of arrhythmias (50% vs 7%, P = 0.014) compared with the control group. In the study group, one patient each had clinically relevant sinus node dysfunction, third-degree AV block, Wolff-Parkinson-White syndrome and atrial fibrillation, and AV nodal reentrant tachycardia. CONCLUSION: Even in the absence of symptoms, patients found to have a single left superior vena cava should be monitored long-term for clinically important arrhythmias.