Primary malignant cardiac tumours are extremely rare. The authors report a case of primary cardiac lymphoma nine years after implantation of a double leaflet mitral valve prosthesis. Malignant lymphoma is a haematological form of sarcoma. Exceptionally rare, it is a tumour of the immune system occurring principally in immuno-depressed patients. It typically presents as a nodular or diffuse myocardial infiltrate explaining its clinical expression as cardiac failure and atrioventricular block. In view of the usual degree of infiltration, surgery is rarely possible. Survival after "pure" medical therapy (chemotherapy alone or associated with radiotherapy) is 6 to 8 months after diagnosis. Dacron has been implicated in the pathogenesis of primary cardiac sarcoma. Oppenheimer demonstrated experimental induction of sarcoma in the rat by subcutaneous implantation of polymers. In conclusion, although primary cardiac lymphoma is a rare condition, it should be considered, as with thrombosis, a possible differential diagnosis of acute dysfunction of cardiac valvular prostheses.