The Holter monitor, the most frequently used diagnostic test in patients with syncope, is nondiagnostic in over 90% of cases. This study sought to determine the impact of a new noninvasive device, the cardiac loop electrocardiographic (ECG) recorder, after Holter monitoring in 57 patients with unexplained syncope. All patients underwent a standardized evaluation protocol and were the monitor for up to 1 month. In 14 patients, loop recording definitively determined whether an arrhythmia was the cause of symptoms (diagnostic yield 25%; 95% confidence intervals 14 to 38%). Diagnoses included unsuspected ventricular tachycardia (1 patient), high grade atrioventricular block (2 patients), supraventricular tachycardia (1 patient), asystole or junctional bradycardia from neurally mediated syncope (3 patients) and normal cardiac rhythms (the remaining 7 patients). Follow-up of all patients diagnosed as having nonarrhythmic syncope by loop recording showed that none of these patients died suddenly. Cardiac loop ECG recording is an important new diagnostic test in patients with syncope unexplained by Holter monitoring.