BACKGROUND: Unipolar electrogram was reported to be useful for localization of manifest accessory pathway conduction during surgical or transcatheter ablation. However, it is not clear whether the unipolar electrogram would also be useful for localizing the origin of idiopathic left ventricular tachycardia (ILVT) in which pace mapping, activation time and recording of Purkinje (P)-potential have been used for guiding the successful ablation. METHODS: In patients who underwent catheter ablation for ILVT, bipolar and unipolar electrograms were recorded at the sites of current delivery. We analysed the time from P-potential to QRS onset (P-QRS time), time from local ventricular electrogram to QRS onset (V-QRS time) and the morphology and slope of rapid downstroke of unipolar electrograms (Uni-slope) during induced ILVT both at successful and unsuccessful sites. RESULTS: In 14 consecutive patients (11M/3F, mean age 29.3) with ILVT and successful ablation, QRS morphology of ventricular tachycardia was of right bundle branch block (RBBB) with left axis deviation and right axis deviation in 11 and 3 patients, respectively. The average number of current delivery was 4.5 (range 2-12). P-potential was observed in 10/14 (71%) successful sites and 37/47 (79%) unsuccessful ablation sites. The morphology of unipolar electrogram was QS pattern in 12 and QrS pattern in 2 successful sites but rS pattern was not observed at successful sites. P-QRS time was 26.5+/-12.4 and 26.6+/-14.9 msec (p=ns), V-QRS time 3.9+/-7.7 and 0.2+/-8.9 msec (p=ns), Uni-slope 7.1+/-3.1 and 7.3+/-4.5 mV/10 msec (p=ns) at successful and unsuccessful sites, respectively, showing no significant differences between successful and unsuccessful sites. CONCLUSIONS: The slope of rapid downstroke in unipolar electrogram was not useful as a guide for localization of successful ablation in patients with ILVT. However, absence of initial 'r'wave in unipolar electrogram may be helpful in conjunction with other conventional criteria for successful ablation.