Background: By analyzing stored intracardiac electrograms during spontaneous monomorphic VENTRICULAR TACHYCARDIA (VT), we examined the possible mechanisms of the VT initiation in a group of patients with implantable cardioverter defibrillators (ICDs).Methods: Stored electrograms (EGMs) of monomorphic VTs from at least 5 beats before initiation and after the termination of VT were analyzed. Cycle length, sinus rate, and the prematurity index for each episode were noted.Results: We studied 182 episodes of VT among 50 patients with ICDs. VENTRICULAR premature complex (VPC) -induced episode (extrasystolic initiation) was the most frequent pattern (106; 58%), followed by 76 episodes (42%) in the sudden-onset group. Among the VPC-induced group, VPCs were different in morphology from subsequent VT in 85 episodes (80%).Sudden-onset episodes had longer cycle lengths (377±30ms) in comparison with the VPC induced ones (349±29 ms;p= 0.001). Sinus rate before VT was faster in the sudden-onset compared to the VPC-induced group (599±227 ms versus 664±213 ms;p=0.005). Both of these episodes responded similarly to ICD therapy. There was no statistically significant difference in coupling interval, prematurity index, underlying heart disease, ejection fraction, and ant arrhythmic drug use between the two groups (p=NS).Conclusions: Dissimilarities between VT initiation patterns could not be explained by differences in electrical (coupling interval, and prematurely index; p=NS) or clinical (heart disease, ejection fraction, and antiarrhythmic drugs; p=NS) variables among the patients. Information obtained by the analysis of stored EGMs could be helpful for the understanding of VT electrophysiology.