- Introduction Atrial fibrillation (AF) and atrial flutter (AFL) are clinically and electrocardiographically similar. However, considering significant therapeutic differences, differentiation of these 2 arrhythmias is essential. Our aims were to evaluate the misdiagnosis rate among electrocardiograms (ECGs) interpreted as AF or AFL by internists and to describe the factors that could be responsible for the misinterpretation. Methods We evaluated patients discharged with a diagnosis of AF or AFL from internal medicine wards of a tertiary referral center. The reanalysis of the ECGs was performed by 2 senior cardiologists (1 electrophysiologist), blinded to the primary analysis and patient’s clinical data. Results The ECGs of 44 of 268 (16%) patients were misinterpreted and consisted of: 25 (57%) AFL, 5 (11%) SVT, 7 (16%) sinus rhythm with premature atrial beats and 7 (16%) AF. The baseline diagnosis was correct in 212 of 246 (86%) for AF and 12 of 22 (55%) for AFL, P < 0.001. A significantly higher rate of AFL was misdiagnosed compared with AF [25 of 37 (68%) versus 7 of 219 (3%), respectively; P < 0.001], higher in atypical than typical AFL [16 of 20 (80%) versus 9 of 17 (53%), respectively; P = 0.07]. Reduced quality ECGs was found more often among the incorrectly than the correctly diagnosed ECGs (P < 0.001]. Conclusions ECGs, interpreted as AF or AFL by internists, are often misdiagnosed. AFL was misdiagnosed more often than AF, with atypical more often than typical AFL. Consulting with a cardiologist and applying diagnostic criteria may reduce misdiagnosis.