Background: The aim of this study was to determine the frequency and nature of errors made by computer electrocardiogram (ECG) analysis of Atrial fibrillation (AF) and the clinical consequences of the errors. Methods: A total of 10,279 ECGs were collected and they were automatically interpreted by the built-in ECG software, and then reread by two cardiologists. AF-related ECGs were classified into three groups; overdiagnosed AF (rhythms other than AF diagnosed as AF), misdiagnosed AF (AF diagnosed as rhythms other than AF), and true AF (AF diagnosed as AF by both computer ECG interpretation and cardiologists). Results: There were 1,057 AF-related ECGs from 409 patients and among these, 840 ECGs (79.5%) were true AF. Overdiagnosis occurred in 98 (9.3%) cases (Figure 1A). Sinus rhythm and sinus tachycardia with premature atrial contraction and/or baseline artifact and sinus arrhythmia were commonly overdiagnosed as AF. Heart rate ���60 beats/min and baseline artifact significantly increased the likelihood of overdiagnosed AF. Misdiagnosis occurred in 119 (11.3%) cases where AF was usually misdiagnosed as sinus or supraventricular tachycardia (Figure 1B). The presence of tachycardia and low-amplitude atrial activity significantly increased the likelihood of misdiagnosis of AF. Among the erroneous computer ECG interpretations, 17 cases (7.8%) were not corrected by the ordering physicians and/or repeat computer-ECG interpretation and inappropriate managements of patients were made. Conclusions: Erroneous computer ECG interpretation of AF was not rare. Attention should be concentrated on educating physicians about the ECG appearance and confounding factors of AF along with ongoing quality control of built-in software for automatic ECG interpretation.