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To clarify the mechanisms of ventricular tachycardia (VT) in humans, sixty patients (average age, 43.9 +/- 15.6 years) with life-threatening VT were examined electrophysiologically. Multielectrode catheters were inserted to the heart and programmed electrical stimulation was performed to induce VT. VT was inducible in thirty (50%) of 60 patients. In twenty-nine (93.6%) of these 30 patients VT was induced by ventricular extrastimuli, suggesting that the most important mechanism for the induction of VT is reentry. Background of VT was idiopathic (IDIO) in 36 patients. The remaining 24 patients had underlying heart disease (HD), and only two of them had old myocardial infarction; this was considerably different from that in USA and Europe in which more than 70% of patients with VT have ischemic heart disease. Induction rate VT (IRVT) in IDIO and HD were both 50%. IRVT in 46 sustained VT was 63% and it was significantly higher than that in 14 non-sustained VT (7.1%). The QRS morphology of VT was complete right bundle branch block (RBBB) pattern in 33 patients [with left axis deviation (LAD); 14 and without LAD; 19] and left bundle branch block (LBBB) pattern in 24 patients. IRVT was significantly high in RBBB with LAD (92.9%) compared with RBBB (63.6%), RBBB without LAD (42.1%) and LBBB (37.5%). Both cardiothoracic ratio and ejection fraction had no relationship to IRVT.