Introduction: Inappropriate sinus tachycardia is usually a diagnosis of exclusion. It is generally based on 12-lead ECG, ECG Holter monitoring and ECG eff ort testing. We hereby present a series of cases in which the initial diagnosis was inappropriate sinus tachycardia. Methods: The four patients (3 males) had a history of inappropriate sinus tachycardia lasting between 2 and 7 years, ECG Holter monitoring showed heart rates between 110 and 130 bpm while resting ECG interpreted by cardiologists was considered sinus tachycardia. One of the patients had overt tachycardiomyopathy (LVEF 25%), while two of the patients had borderline forms (upper normal LVEDD with minimally depressed LVEF 50-55%). All four patients were under beta-blockers, one of them was also under ivabradine, without adequate heart rate control. In all of these patients atrial tachycardia was suspected based on discrete changes in p wave morphology at normal heart rates (< 100 bpm). Results: All patients were referred for electrophysiological study; RA mapping revealed atrial tachycardia originating from the right atrial appendage (RAA). Radiofrequency ablation restored normal sinus rhythm in all patients. ECG Holter monitoring and echocardiography during follow-up showed sustained sinus rhythm and normalization of echocardiographic parameters in all 3 patients with tachycardiomyopathy. Conclusions: Atrial tachycardia originating from RAA can mimic inadequate sinus tachycardia. Clinical awareness and thorough evaluation of p-wave morphology changes are required for a correct diagnosis and treatment.