Background: Adults with congenital heart disease (ACHD) are at risk of developing arrhythmias. The purpose of this study is to assess the incidence of arrhythmias encountered on ECG and Holter monitors in ACHD patients. Methods: A review of ACHD patients from 7-04 - 12-07. Data collection included: ECGs, and 24-hour Holter monitor results. Inclusion criteria: age ≥ 18 years, congenital heart disease, ECG and 24-hour Holter monitor. Results: We identified 333 patients with 500 visits. Inclusion criteria was met by 140 patients. ECGs revealed 109 normal and 31 abnormal (22%), with most common abnormalities consisting of ectopy (7%), supraventricular tachycardia (6%), ST abnormalities (4%), junctional rhythm (3%), WPW (2%), pacemaker issues (2%), prolonged QT interval (2%. Holter monitors revealed 43 (31%) normal and 97 (69%) abnormal, including ectopy (66%), ventricular couplets (11%), supraventricular tachycardia (10%), ventricular tachycardia (6%), high grade atrioventricular block (5%), junctional rhythms (4%), pacemaker problems (3%), and WPW (1%). In ACHD with normal ECGs, 69% had abnormal Holters. In ACHD with abnormal ECGs, 71% had abnormal Holters. In 48 patients with multiple visits, 33 (69%) had significant changes on Holter monitors. Conclusions: Abnormal ECGs and/or Holter monitors are present in a significant percentage of ACHD patients. In ACHD patients with a normal ECG, a majprity have abnormal Holters. We strongly recommend that a Holter be performed on all ACHD patients regardless of ECG results. In addition, repeat Holter monitoring can identify significant changes.