To assess the prognostic value of ventricular tachycardia (VT) morphology we studied data from 56 patients (pts) with old myocardial infarctions and documented sustained monomorphic VT. There were 22/56 pts (39%) with a left bundle branch block (LBBB) pattern of the spontaneous VT and 34/56 pts (61%) with a right bundle branch block (RBBB) VT. During a mean follow-up of 34 +/- 11 months, 12 pts died from cardiac causes, four of them suddenly. Total mortality was higher in pts with RBBB-VT (9/34, 26%) than in those with LBBB-VT (3/22, 14%) (p = 0.21), despite a significantly shorter VT cycle length in pts with LBBB-VT (306 +/- 56 ms) than in those with RBBB-VT (344 +/- 76 ms) (p less than 0.05). All pts who died suddenly had a RBBB-VT. During programmed stimulation, VT with the same morphology as the spontaneous VT was induced more frequently in pts with LBBB-VT (off drugs: 7/14, 50%; on drugs: 10/16, 63%) than in pts with RBBB-VT (off drugs: 7/20, 35%, on drugs: 8/28, 29%) (p = ns). In contrast, VT with different or multiple morphologies were induced more frequently in pts with RBBB-VT (off drugs: 13/20, 65%; on drugs: 20/28, 71%) than in pts with LBBB-VT (off drugs: 7/14, 50%; on drugs: 6/16, 38%) (p less than 0.05). Our data show that there is a higher risk of cardiac and sudden death in pts with RBBB-VT than in those with LBBB-VT after myocardial infarction, despite a shorter cycle length of the spontaneous VT in patients with LBBB-VT.