It is widely accepted that patients presenting with acute anterior myocardial infarction and acute onset of right bundle branch block should be prophylactically paced in contrast with those who have a chronic bundle branch block. The admitting physician is faced with the dilemma of how to act if the age of this conduction disturbance is unknown. This problem has further intensified in recent years, with the introduction of thrombolytic treatment, where insertion of a central vascular line is associated with increased morbidity. The objectives of this study were to define clinical or electrocardiographic parameters that may help the admitting physician to decide whether patients presenting with an anterior wall myocardial infarction and a right bundle branch block of unknown age should be prophylactically paced. We examined prospectively the in-hospital clinical course of 39 consecutive patients presenting with an acute myocardial infarction in whom the age of a right bundle branch block upon admission was unknown (group C, n = 39) and compared with two similar groups of patients who presented with an acute right bundle branch block (group A, n = 38) and with a known chronic right bundle branch block (group B, n = 22). Thirty-three patients (33%) died, with cardiogenic shock being the leading cause of death in the entire population. Prophylactic pacing, which was carried out in 66% and 54% of patients in groups A and C, respectively, did not reduce mortality rates. No clinical or electrocardiographic variables on admission were predictive to support prophylactic pacing in group C. In 10 of 46 (22%) patients who were prophylactically paced with a transvenous electrode, the following complications attributed to the procedure were detected: (1) either rapid sustained ventricular tachycardia (during implantation) that was unresponsive to overdrive pacing, or ventricular fibrillation necessitating electrical defibrillation (4 patients); (2) recurrent episodes of rapid nonsustained ventricular tachycardia, which stopped only after the pacemaker was turned off (1 patient); (3) complete AV block (1 patient); (4) fever appearing on the third or fourth day after implantation (3 patients); and (4) a large hematoma in the groin in 1 patient who was treated with thrombolysis shortly before pacemaker electrode insertion. Thus, the complications of transvenous temporary pacing in the era of thrombolysis may outweight any theoretical advantage.