Thymic cysts are rare and almost always asymptomatic. The authors report the case of a 45 year old woman with a thymic cyst diagnosed after recurrent right sided heart failure resulting in signs suggestive of adiastole, regressing after "pleural" (mainly cystic) aspiration and diuretic therapy without any morphological or functional changes on Doppler echocardiography. This report concerns a rare tumour, with an exceptional volume (2 litres) extending down the cardiac borders and causing cardiac compression. It illustrates the diagnostic difficulty of a pathology with an unusual clinical presentation, despite complementary investigations including CT scan and MRI, very sensitive in this type of problem. A complete cure was obtained by total surgical ablation.