This study aimed to evaluate the prevalence of preexcitation and possible pseudo-preexcitation among patients undergoing complex congenital heart surgery at a single center. During a 6-year period (2007-2013), the study retrospectively reviewed the electronic medical records and electrocardiograms (ECGs) of 98 patients who had undergone complex cardiac surgery at the University of Iowa Children's Hospital. The presence and timing of delta waves or short PR interval were evaluated as well as the results of testing for the presence of an accessory pathway. The ECG of 17 patients (17.3%) showed delta waves. Eight of these patients also had a short PR interval. A significant majority of the 17 patients (n = 13) with delta waves showed hypoplastic left heart syndrome (HLHS), with atrioventricular canal defect and Ebstein's anomaly also represented. Two patients with delta waves had adenosine testing that showed antegrade conduction block, greatly reducing the likelihood of an accessory pathway. One patient with delta waves had an electrophysiology study demonstrating an accessory pathway, and the remaining patients had no additional testing for an accessory pathway. For 12 patients, the findings showed a short PR interval on at least one ECG with no evidence of delta waves. Delta waves were commonly seen in patients with HLHS and less commonly in patients with other diagnoses. The high prevalence of preexcitation in combination with our testing confirmed the lack of an accessory pathway in the presence of a delta wave, suggesting that pseudo-preexcitation may be a frequent finding. A short PR interval also was seen frequently in patients exhibiting HLHS, with and without the presence of a delta wave.