Secundum atrial septal defect (ASD) is the most common type of ASD within the spectrum of congenital cardiac abnormalities in children. In this retrospective study the data on 75 patients with a pure secundum ASD were analysed. The male/female ratio was 1:1,3. This is less than the reported male / female ratio, of 1:2. The symptoms most often found were shortness of breath, cough and susceptibility to fatigue. An ejection systolic heart murmur was found in the majority of the patients. A fixed split second heart sound was only found in 49% of patients. This could have been the result of incomplete documentation, inexperienced clinicians, a fast heart rate or the fact that a split second heart sound is not always present in patients with ASDs. On the chest radiograph the majority of patients presented with right ventricular enlargement, right atrial enlargement and increased pulmonary blood flow. A relatively unexpected finding on echocardiography was left atrial enlargement. This is usually caused by volume overload in the presence of large ASDs. Another unexpected finding was the high number of left QRS-axis deviations (10%) and indeterminate QRS-axis deviations, left rotation (7%) present on the electrocardiogram. This finding is much higher than the reported 3.8% of left QRS-axis deviation present in patients with secundum ASDs. The only explanation for this phenomenon is that during the embryological development of the atrial septum, not only does a secundum ASD develop, but the endocardial cushion becomes involved, giving rise to abnormal conductive pathways.