Marastic endocarditis is a rare clinical condition described in cases of cancer or other severe inflammatory diseases. The authors report the case of a young patient in good general condition, admitted after a cerebro-vascular accident. Investigations showed an isolated mitral valvular mass on transoesophageal echocardiography which, after unsuccessful medical therapy, was operated. It was, in fact, a case of marastic endocarditis, and a pulmonary tumour was discovered one month after surgery. The bronchopulmonary adenocarcinoma had remained infraclinical beforehand. The advances in echocardiographic imaging will probably lead to an increase in such cases of early diagnosed thrombotic non-bacterial endocarditis (ETNB. This case suggests that it is justified to carry out an aetiological investigation of thrombotic non-bacterial endocarditis in all cases of isolated mitral valve masses.