We report on a 40-year-old woman referred for evaluation of a cardiac murmur and dyspnea on exertion. The electrocardiogram (ECG) showed incomplete right bundle branch block, and echocardiography revealed a large atrial septal defect (ASD, ostium secundum type) with dilated right-sided heart chambers. At cardiac catheterization, a large left-to-right shunt (78% of the pulmonary blood flow) was found, and surprisingly, the additional diagnosis of anomalous origin of the left coronary artery from pulmonary artery (ALCAPA) was established. After ASD closure and left coronary artery ligation with implantation of a vein graft to the left anterior descending artery, she had an uneventful 18-years follow-up. We discuss the interaction of the two associated conditions, and based on the herein reported unusual combination, we highlight typical features of non-invasive examinations including auscultation, ECG, and echocardiography in adult patients with ALCAPA.