Cardiac hydatid cyst is a rare condition. The diagnosis is difficult and is based on a series of findings amongst which hydatid serology and cardiac imaging play important parts. The values and limitations of echocardiography, coronary angiography and CT scanning are well known. Nuclear magnetic resonance imaging is a recent technique which theoretically should provide valuable diagnostic information. The authors report a case in which this technology, though confirming the presence of a polycystic intrapericardial mass, did not show the true extension of the disease.