Introduction: The ostium of coronary sinus has been described as a site of origin of atrial tachycardia. The goal of this study was to characterize the electrocardiographic and electrophysiologic features of focal atrial tachycardia originating from the coronary sinus ostium. Methods: Four patients (age 62짹 13 years, 2 women) undergoing electrophysiologic study and radiofrequency ablation for atrial tachycardia were included in this study. Results: The earliest activation site was located at the coronary sinus ostium -25짹7 ms ahead of the onset of P wave. Atrial tachycardia was induced by atrial extrastiumulation in 3 and ventricular extrastimulation in 1 patient. Isoproterenol infusion was required for induction in 3 patients. Polarity of P wave was negative in all inferior and late precordial leads (V3-V6). Rapid ventricular pacing during the tachycardia produced ventriculoatrial dissociation in all patients. In 3 patients atrial tachycardia was terminated by adenosine. Radiofrequency ablation that targeted the earliest atrial activation site (-48짹23 ms ahead of the onset of P wave, two in upper lip of ostium and two in lower lip of ostium) was effective in all patients. Conclusion: Focal atrial tachycardia from coronary sinus ostium had characteristic morphology of P wave, was sensitive to adenosine and long-term success can be achieved by catheter ablation.