Atrial septal defect of the ostium secundum type is almost always sporadic, with multifactorial inheritance. We report a case of a family in which atrial septal defects of the ostium secundum type with atrioventricular conduction defects were observed in four generations: five proven cases and two probable cases. Ten families have been reported as showing atrial septal defect of the ostium secundum type and atrioventricular conduction defects with dominant inheritance. Moreover, in our family the transmission model can also confirm autosomal dominant inheritance. We believe that cardiologists should take a familial approach to atrial septal defect of the ostium secundum type for both research and genetic counseling purposes. In a patient with ASD II, the AV conduction defects should prompt a detailed family history and clinical evaluation of first-degree relatives.