Introduction: It is very important to diagnose and treat supraventricular arrhythmias with atypical manifestations. The importance of the approach of such arrhythmias consists of: • detecting the forms potentially malignant, not visible on the surface electrocardiogram; • evolution during time to malignancy of benign arrhythmias with accessory pathways (AP) as substrate; • arithmological interest in the onset mechanism and mode of manifestation of the tachycardia during exploration by electrophysiological study (EPS); • highlighting rare or successive closure of the circuit in the case of reentrant arrhythmias. Methods: In the Institute of Cardiovascular Medicine Timișoara was hospitalized, between 1998 and 2014, a total of 3,700 patients with supraventricular tachyarrhythmia (SVT), of which a number of 90 patients (2.43%) had abnormal tachyarrhythmia. Patients with Wolff -Parkinson-White syndrome (WPW sd.), atrioventricular nodal reentrant tachycardia (AVNRT) atrial flutter (AFL), atrial tachycardia (AT) were included in the study. Those arrhythmias had as common feature the increased vital risk. Each patient with abnormal pattern arrhythmia underwent electrophysiological study and radiofrequency current catheter ablation. Results: The changes revealed by EPS were: block in the 4th phase of depolarization in the AP, WPW sd. intermittent or absent on the ECG, with short refractory period – 5 p (5.55%); WPW sd. masked by a predominant conduction through a hyper-permeable AV node and/or inter-atrial conduction disturbances – 28 p (31.11%); evolution to malignancy of WPW sd. documented as benign – 7 p (7.77%); changes to malignancy caused by structural heart disease – 10 p (11.11%); retrograde block in the 4th phase of the concealed Aps – 1 p (1.11%); degeneration into VF of SVT, normal heart – 2 p (2.22%); AVNRT with 250 – 280 b/min. and shock – 2 p (2.22%); AFL with 1:1 AV conduction and shock – 8 p (8.88%); AT with trigger mechanism – 3 p (3.33%); complex tachycardia mechanism, substrate combinations (multiple APs, Kent, Mahaim) – 24 p (26.66%). Conclusions: Even the apparently benign forms of SVT can be life threatening: WPW sd. cannot be labeled benign forever; AVNRT can be life threatening by heart rate over 250 b/min. or can degenerate into ventricular fibrillation; AFL with 1:1 atrioventricular conduction and consecutive diastolic cardiogenic shock. Successful radiofrequency ablation of these arrhythmias fully solves the problem of sudden death. Complex mechanism tachycardia can have difficult diagnosis and can be solved by successive ablations of the arrhythmogenic substrate, followed each time by electrophysiological study. Some atypical forms of arrhythmia events can be listed among electrophysiological curiosities and can be potentially better understood in terms of mechanism by cellular electrophysiology.