A case of atrial ectopic tachycardia in an infant probably caused by a small tumor in the interatrial septum is presented. The arrhythmia was successfully treated with amiodarone. Several histologic abnormalities are known to cause AET, but an intracardiac tumor detectable by two-dimensional echocardiography is extremely rare. AET -especially in childhood-as well as cardiac tumors show a tendency to regression, so that even in this case medical treatment should be the first therapeutic choice. Interventional or surgical treatment is required just for those patients with AET this is resistant to medical treatment or reoccurs after finishing the medical treatment.