•A large cardiac cyst was discovered in an extremely rare location: the IVS.•Pre- and postcontrast imaging is essential evaluation of a cystic mass.•When identifying cyst, malignancy infection must be considered.•The benefit excising unclear asymptomatic older patient. Cardiac cysts are occurrence adults, with most being congenital, simple morphology, benign.1Feigin D.S. Fenoglio J.J. McAlliste...