We present and discuss the case of a 26-year-old man suffering from Churg-Strauss syndrome (CSS). In our patient the disease severely affected both the heart and the lungs. Left ventricular dysfunction and intracardiac thrombosis, probably secondary to the cardiotoxic effect of eosinophils, were revealed by echocardiography. The risk of thrombosis was additionally increased by the coexistence of the factor V Leiden gene mutation. An opportunely established diagnosis allowed institution of an appropriate pharmacotherapy and subsequently to obtain a significant abatement of the clinical symptoms as well as the laboratory manifestations of the disease.