Determination of ventricular and supraventricular arrhythmias with left bundle branch block morphology (LBBB) seems to be one of the most complex diagnostic tasks when P waves are not clearly identifiable on 12-lead ECG. Methods Twenty-eight consecutive patients with wide QRS premature beats and LBBB morphology identified by 24-hour and long-term ECG monitoring were enrolled in the study. Fourteen patients had premature atrial contractions (PAC) and fourteen patients had premature ventricular contractions (PVC) during sinus rhythm. For each patient 10 typical single wide QRS complexes in 12 ECG leads were analyzed. The duration of QRS complexes, time intrinsic deflection (TID) and electrical axis of the heart were determined automatically in 12 leads ECG. ROC analysis with optimal cut-off value level evaluation as well as calculation of sensitivity (SV), specificity (SP) and diagnostic accuracy (ACC) were performed for qualitative and quantitative assessment of diagnostic value of PAC and PVC amplitude and temporal characteristics. Results The results of study found out that V5, V1 leads were more informative for assessment of QRS complex morphology and duration while V1 and V4 leads were more informative for TID calculation in differential diagnosis of wide QRS complexes arrhythmias with LBBB morphology. PAC with aberrant conduction had the following characteristics: TID < 68 ms in V1 lead and TID < 62 ms in V4 lead; QRS < 136 ms; transition zone in the leads V5, V6; QS, rS or RS complexes (with S> R) with low-amplitude r/R waves in leads V1 - V4; electric axis of the heart less than 80° in standard leads. In all cases PVCs were characterized by QRS > 160 ms and TID > 88 ms in V1-V6 leads. Conclusions New various criteria can improve differential diagnosis of wide QRS complexes with LBBB morphology were identified in this study.