Transcatheter occlusion of atrial septal defects (ASD) is currently being investigated as an attractive alternative to surgical correction. Thromboembolic events are rare in both techniques. However, we report a case of massive systemic embolization and residual left atrial thrombus after secundum ASD transcatheter closure by the ASDOS device (Atrial Septal Defect Occlusion System, Dr. Ing Osypka Corporation, Germany). The patient was successfully treated by femoral embolectomies, surgical removal of the device and closure of the ASD without a patch. No thrombophilia was found on subsequent exploration. Transcatheter ASD closure with the ASDOS device may therefore expose the patient to severe embolic complications. Further evaluation is needed before this technique can be safely recommended.