Objective: The present article was motivated by the frustrating experiences with four patients who underwent surgical treatment of ostium secundum atrial septal defect (ASD-II) and who died in extremely dramatic circumstances.Method: This is a retrospective study based on clinical data. The bibliographical research included: paradoxical thromboembolism (cerebral, pulmonary or mesenteric), central nervous system vascular malformations and anomalous vena cava connections. This research was based on possible events, unexpected and catastrophic, that could have directly caused the patients' deaths.Results: All patients were female. The operations were performed under cardiopulmonary bypasses with less than 20 minutes of aortic cross-clamping. The causes of death were: non-occlusive intestinal ischemia, rupture of a cerebral aneurysm of the anterior communicating artery, cor pulmonale associated with pulmonary arterial hypertension and thromboembolism and probable cerebral thromboembolism in a child that needed to be reoperated to correct anomalous inferior vena cava drainage into the left atrium.Conclusion: Even being technically easy to treat, death due to atrial septal defects in extremely dramatic circumstances is possible, so "Do not underestimate atrial septal defects in congenital heart surgery". Descriptors:Heart septal defects, atrial, surgery, complications. Heart defects, congenital, surgery, complications. Extracorporeal circulation.Eventos catastróficos associados ao tratamento da comunicação interatrial tipo ostium secundum: razões para não se subestimar este tipo de cardiopatia congênita Catastrophic events associated to the surgical treatment of ostium secundum atrial septal defects. Reasons for not underestimating this type of congenital cardiopathy 392 EVORA, PRB ET AL -Catastrophic events associated to the surgical treatment of ostium secundum atrial septal defects. Reasons for not underestimating this type of congenital cardiopathy Braz