Abstract Aims: To analyze the diagnostic yield of 24-h electrocardiographic monitoring (Holter) as a function of cardiac disease and electrocardiography (ECG) status in patients with unexplained syncope. Methods: Patients were considered as having unexplained syncope when a careful clinical history, physical and neurological examination, 12-lead ECG and testing for orthostatic hypotension were negative. All patients underwent 24-h Holter. According to the type of arrhythmias that occurred during monitoring, Holter was classified as: (1) normal, in the absence of significant arrhythmias; (2) abnormal, in the presence of significant (e.g. atrial fibrillation) but non-diagnostic arrhythmias; or (3) diagnostic, in the presence of serious arrhythmia (e.g. complete AV block) with simultaneous symptoms. Clinical features, ECG and echocardiography were used to stratify patients and identify those in whom electrophysiological studies (EPS) were indicated. Results: 140 consecutive patients with unexplained syncope underwent 24-h Holter and 44 of them (31%) had non-diagnostic arrhythmias. These occurred more frequently in patients with a positive cardiac history or an abnormal ECG (31/73, 42%), compared to patients without these features (13/67, 19%, p<0.05). Nine of the 140 patients (6%) had serious arrhythmias during Holter considered as diagnostic because of the presence of simultaneous symptoms. These occurred exclusively in patients with a positive cardiac history or an abnormal ECG (9/67, 12%). All diagnostic arrhythmias recorded during Holter occurred in patients in whom EPS was beforehand deemed indicated based on the presence of structural heart disease and/or an abnormal ECG. Conclusions: When restricted to high-risk patients, 24-h Holter remained a useful initial step in the evaluation of unexplained syncope with a symptoms and arrhythmias correlation occurring in 12% of patients. When performed first, this procedure might avoid doing EPS.