Unspecified atrial structures can be a hazard to interventional device closure of atrium secundum defects in adult patients. A 48 year old man was scheduled for device closure of a secundum atrial septal defect (ASD). On transoesophageal echocardiography an echogenic, round structure (diameter 5 mm) was detected in the centre of the right atrium (panel A). In some multiplane transoesophageal echocardiogram views a right atrial thrombus was suspected (panel B). To clarify the aetiology of this structure a magnetic resonance imaging study was performed (panel C, transversal image). On sagittal cuts with and without gadolinium an abnormal right atrial fold (not a thrombus and not a tumour) was identified as causative (panel D, sagittal image). The ASD was then closed with an Amplatzer septal occluder.