Ostium primum defect is a congenital malformation involving atrial septum contiguous with atrioventricular valve annulus; it is accompanied by abnormalities in the development of the endocardial cushions, often resulting in associated atrioventricular valves malformations. Few cases have been reported in adulthood because these patients frequently come to medical attention at an earlier age when symptoms such as dyspnea, fatigue, cyanosis, and tendency to underweight occur. Various factors affect the timing of clinical presentation, but the most important is the degree of mitral/tricuspid insufficiency; when valve regurgitation remains moderate, the appearance of symptoms may be delayed for decades. In adult patients, deterioration of clinical status and death are mainly due to the development of arrhythmias or heart block. We present the case of a 67-year-old patient, without previous cardiovascular events, with a new onset of atrial fibrillation, who developed dyspnea and fatigue; echocardiography showed a large interatrial defect localized in the basal portion of the septum, associated with anterior mitral valve cleft and moderate regurgitation. The patient underwent surgical closure of the defect (intraoperatory measures 1,9 × 3 cm) with autologous pericardium patch; a permanent epicardial pacemaker was implanted for the development of complete atrioventricular block in the early postoperative period.