The purposes of this study were to document the findings on prolonged electrocardiographic monitoring in patients with syncope, correlate these findings with symptoms during monitoring, and define the prognostic importance of these findings. A total of 235 patients with syncope in whom a cause was not assigned or suggested by the initial history, physical examination, and electrocardiography were studied. Although a variety of arrhythmias were documented during monitoring, the vast majority of arrhythmias were of short duration and did not produce symptoms. At two years, the patients with frequent or paired premature ventricular contractions (PVCs) when compared with patients with rare PVCs, had a higher incidence of sudden death (18.2 percent versus 4.0 percent; p less than 0.001) and higher overall mortality (28.3 percent versus 10.8 percent; p less than 0.003). Similarly, patients with ventricular tachycardia, when compared with patients with rare PVCs, had a higher incidence of sudden death (18.7 percent versus 4.0 percent; p less than 0.0001) and higher overall mortality (36.5 percent versus 10.8 percent; p less than 0.00001). Cox regression analyses revealed that frequent or repetitive PVCs and sinus pauses were independent electrocardiographic predictors of sudden death and mortality in patients presenting with syncope. Therefore, patients with syncope and frequent or repetitive ventricular ectopy or sinus pauses constitute a high-risk subgroup and may be candidates for more extensive diagnostic evaluation.