We report the case of a 45-year-old Caucasian male with negative cardiovascular history, heavy-smoker, who was referred to our Cardiology Unit for recurrent inexplicable tachycardia. Chaotic atrial tachycardia with intermittent fibrillation was observed at ECG, whereas a smoothed mass (approximately sized 8 × 8 cm) was unexpectedly found at echocardiography likely infiltrating the right atrial wall. Multi-detector computed tomography confirmed the mediastinal mass and the digital post-processing clearly identified its anatomic characteristics and invasivity. This study demonstrates that recurrent and refractory atrial arrhythmias can be early signs of cardiac infiltrating mediastinal masses. The combined approach by echocardiography and computed tomography was confirmed to provide precise anatomical and functional characteristics of the arrhythmogenic disease in this patient.