Atrial septal aneurysm (ASA) has rarely been described in children. In this study, we evaluated the incidence and natural course of this anomaly in children. ASA was found in 30 patients (1%); 16 patients had type 1R and 14 patients had type 2L ASA. Twenty patients with ASA were followed: in four the ASA disappeared and in three patients with type 2L an atrial septal defect (ASD) developed during follow-up. The most common associated lesion was patent foramen ovale (PFO). We conclude that ASAs in children are not rare lesions, and that these aneurysms, particularly type 2L aneurysms, should be followed for occurrence of ASD.