Arsenic trioxide is an effective treatment for patients with acute promyelocytic leukaemia (APL) who have relapsed from or are refractory to all trans-retinoic acid and anthracycline chemotherapy. Cardiac effects observed include electrocardiographic changes such as QTc prolongation, T-wave abnormalities, torsades de pointes and sudden death. We describe a case of a man, 76 years old, who was admitted to our department for dyspnoea in APL in treatment with arsenic trioxide. Chest radiograph illustrated an enlarged cardiac silhouette and bilateral pleuric effusion and the ECG evidenced QT prolongation. The patient was also submitted to transthoracic echocardiography that revealed moderate pericardial effusion without signs of cardiac tamponade and a normal biventricular function. This condition was considered to be associated with arsenic trioxide polyserosit and the drug therapy was immediately discontinued and steroid drugs started. After 2 weeks of arsenic trioxide therapy suspension there was evidence of complete resolution of pericardial and pleuric effusion and the ECG showed normal QT interval.