Introduction: Inappropriate sinus tachycardia (IST), a clinical syndrome with distressing symptoms, is characterized by paroxysmal or persistent sinus tachycardia in the absence of any identifiable primary etiology. Sinus node modification is not first line therapy given frequent recurrence and high complication rates. While medical therapy may be effective, symptoms frequently persist. Our objective is to describe the range of arrhythmias encountered during ablation in a cohort of IST patients refractory to medical therapy and to further the understanding of the etiology and mechanism of IST. Methods: We present a retrospective review of 33 patients who underwent sinus node modification for the treatment of therapy-refractory IST. Results: Arrhythmias included IST in 33 patients (100%), atrial tachycardia or frequent premature atrial contractions in 16 patients (48%), atrial fibrillation in 7 patients (21%), typical and atypical atrial flutter in 13 (39%) and 2 patients (6%), respectively, Wolff-Parkinson-White syndrome in 3 patients (9%), AV nodal reentry tachycardia in 3 patients (9%), and ventricular tachycardia or premature ventricular contractions in 4 patients (12%). Accelerated atrioventricular junctional (AVJ) tachycardia was present in 7/33 patients (21%), was refractory to medical therapy and caused disabling symptoms in all seven patients. Patients with AVJ tachycardia required extensive ablation at multiple sites, and 3/7 patients became pacemaker dependent. Conclusion: In patients undergoing SAN modification for IST, multiple arrhythmias were encountered. Accelerated AVJ tachycardia was frequently encountered and required extensive ablation of up-regulated tissue in the AVJ to improve symptoms. Thus, the pathologic process underlying IST may not be limited to the sinus node.