To characterize the events that precede and precipitate sudden cardiac death (SCD), the long-term electrocardiograms of 27 patients who had SCD while being monitored were analyzed. In 20 patients, SCD was associated with ventricular tachyarrhythmias (ventricular tachycardia [VT]/ventricular fibrillation [VF]) and in 7 it was associated with bradyarrhythmias. Seventeen of the patients were men and 10 were women. Twenty-one patients had coronary artery disease, 2 had idiopathic dilated cardiomyopathy, 2 had mitral stenosis and 1 patient had mitral valve prolapse. Four patients with VT/VF had a previous nonfatal cardiac arrest. In the 20 patients with tachyarrhythmia-related SCD, 3 or more VT beats always preceded degeneration to VF. In 5 patients, the frequency or complexity of ventricular arrhythmias increased in the hour before SCD. In 11 of 20, there was a 20% or greater increase in underlying heart rate in the hour before SCD. The R-on-T phenomenon was observed in 4 patients. The long-short phenomenon initiated VT/VF in 2 patients. Only 2 patients with VT/VF were resuscitated. No patient with bradyarrhythmia -related SCD had manifest atrioventricular block or bundle branch block. Two of 7 patients had an episode of nonsustained bradycardia in the hour before arrest. No patient was resuscitated. In conclusion, VT that degenerates into VF is the most common arrhythmia associated with SCD. VT/VF is frequently preceded by an increase in heart rate and complex ectopy. VT is most often initiated by late ventricular premature complexes. Twenty-five percent of patients who have SCD have associated bradyarrhythmias that may occur without premonitory events.