Five patients with recurrent tachycardias exhibiting right bundle branch block with left axis deviation were referred for investigation. In each case, a supraventricular mechanism was suspected. During sinus rhythm the QRS morphology and axis (-10 to +60 degrees) and HV intervals were normal. Tachycardia was initiated by timed ventricular premature stimuli in 4 patients, rapid ventricular pacing in 3 patients and rapid atrial pacing in 2 patients. The tachycardia cycle length varied from 275 to 380 ms with right bundle branch block and a leftward axis change of 30 to 125 degrees at the onset of the tachycardia. The HV interval ranged from +15 to -20 ms. In each patient ventriculoatrial dissociation occurred spontaneously or could be induced. All tachycardia could be terminated or greatly slowed by calcium antagonists. These data are consistent with an unusual reentrant mechanism of tachycardia located in the posterior fascicle of the left bundle branch.