It is still controversial whether the arrhythmias in acute pericarditis are of myocardial or pericardial origin. The aim of the present study was to investigate the occurrence of arrhythmias and conduction disorders in patients with acute pericarditis with no endomyocardial biopsy evidence of myocarditis (group 1: 40 patients, 65% males, mean age 45.6 +/- 15.7 years, mean heart rate [HR] 98.7 +/- 22.2 beats per minute) in comparison to endomyocardial biopsy proven acute myocarditis/perimyocarditis (group 2: 10 patients, 3/10 with perimyocarditis, 70% males, mean age 46.1 +/- 15.8 years, mean heart rate 76.7 +/- 33.1 beats per minute). At the initial assessment all patients underwent comprehensive clinical work-up including echocardiography, cardiac catheterization, and endomyocardial biopsy. In all patients biventricular endomyocardial biopsy was performed using standard femoral approach and Schikumed 7 F or 8 F bioptomes. Tissue samples were stained by H & E, v. Gieson and independently reviewed by two cardiac pathologists. In addition immunohistochemistry and immunocytochemistry were performed, and only patients fulfilling Dallas and World Heart Federation criteria were selected for group 2. Comparative analysis of electrocardiograms and 24-hour Holter recordings at initial presentation revealed in group 1 vs group 2 significantly less frequent paroxysmal supraventricular tachyarrhythmias (5% vs 40%), and ventricular fibrillation (0 vs 20%), in contrast to atrial fibrillation that occurred more often (20% vs 0) (all p < 0.05). Furthermore, in the group 2 one patient died due to VF and two patients underwent ICD implantation. Low voltage (40% vs 30%) and ST/T wave changes (47.5% vs 30%), as well as the incidence of the II degree AV block (5% vs 0) and complete AV block (2.5% vs 10%) were not significantly different between the groups. In conclusion, patients with pericarditis and no endomyocardial biopsy indications of myocarditis had significantly less often life threatening rhythm disorders in contrast to patients with endomyocardial biopsy proven acute myocarditis/perimyocarditis. On the contrary, incidence of transitory atrial fibrillation was higher in acute pericarditis, than in myocarditis.