A 67-year-old male patient (height 165 cm; body weight 88 kg), without previous cardiological history, was admitted to our emergency room for paroxysmal nocturnal dyspnea. The patient had reduction of functional capability and dyspnea (New York Heart Association functional class III) in the preceding months. He was on chronic therapy with enalapril for systemic hypertension. Electrocardiogram on admission showed atrial fibrillation with ventricular rate response of 100 beats/min, right bundle branch block and diffuse repolarization abnormalities. Physical examination revealed a 2/6 L holosystolic murmur over the aortic area, a fixed split of the second heartbeat and mild signs of pulmonary congestion. Transthoracic echocardiography showed normal size of the left ventricle, which was hypertrophic and with normal ejection fraction. Color-Doppler evaluation revealed left to right shunt in the basal portion of the interatrial septum without the involvement of the interventricular septum [Figure 1] and moderate mitral valve regurgitation. Right atrium was dilated, right ventricle was hypertrophic and mildly dilated (telediastolic diameter 49 mm) with moderate to severe tricuspid regurgitation. Systolic pulmonary artery pressure was 40 mmHg. Chest X-ray showed posterior bilateral pleural effusion, diffuse thickening of the interstitial peribronchial vascular tissue and enlargement of the cardiac shadow. For further evaluation, a transesophageal echocardiography was performed, confirming an atrial septal defect, ostium primum type, with left-to-right shunt [Figure 2]. Mitral valve appeared dysmorphic, fibrocalcific (compatible with a cleft of mitral valve's anterior leaflet) with moderate regurgitation. Subsequently, coronary angiography revealed no coronary artery disease. The patient underwent surgical closure with autologous pericardium patch (intraoperative measurement of the defect was 1,9 x 3 cm, localized in the caudal portion of the septum). During surgery, the patient developed complete atrioventricular block requiring permanent epicardial pacemaker implantation. Postoperative transthoracic echocardiography showed no residual atrial communication, with a slight reduction of the right sections dimensions and mild mitral regurgitation.