The American Heart Association (AHA) currently recommends the precordial thump as the initial maneuver in treatment of ventricular tachycardia (VT) and monitored ventricular fibrillation (VF). These recommendations are based largely on anecdotal reports of successful "thump-version" of asystole, VF, and VT. The Milwaukee County Paramedic System follows the AHA guidelines in the treatment of VT and VF. The precordial thump is included in the advanced cardiac life support (ACLS) paramedic training program, and has been used in our approach to the pulseless, nonbreathing patient. During an eight-month period, 50 pulseless, nonbreathing patients received precordial thumps during ACLS resuscitative attempts. Twenty-seven patients who developed monitored VT and 23 patients with monitored VF were thumped. Three of 27 patients (11%) with VT were thumped into a supraventricular rhythm, 12 of 27 patients (44%) remained in VT, and 12 of 27 patients were thumped from VT into more malignant rhythms: three, into asystole; eight, into VF; and one, into an idioventricular/electromechanical dissociation rhythm. A total of 23 patients were thumped without effect. Subsequently, using countershock and medications, 12 of these 23 patients were successfully resuscitated. In the prehospital setting the precordial thump is usually not beneficial, and may be detrimental. Thus its use as the initial maneuver in treating the cardiac arrest patient with VT or VF in this setting cannot be supported. The presence of acidosis and hypoxia may explain why prehospital precordial thump responses differ from those seen in the hospital environment.