For four months a 28-year-old man had been suffering from a sensation of pressure in his head when lying down, swelling of the face, exertional dyspnoea and weight gain of 9 kg. Biochemical tests and chest radiograph were unremarkable. Echocardiography revealed a 3 x 5 cm mass in the right atrium, extending into the superior vena cava whose lumen was narrowed to a few millimetres. Computed tomography and magnetic resonance imaging were suggestive of a malignant tumour. Bone marrow biopsy as well as transvascular and mediastinoscopically obtained biopsies failed to provide a pathological diagnosis. Exploratory thoracotomy uncovered a nonresectable chloroma surrounding both venae cavae. It had penetrated into the right and left atria, as well as the pulmonary veins. A Gore-tex bypass was placed between the innominate vein and right atrial appendage. The tumour was identified as a local manifestation of acute myeloid leukaemia. Treatment with idarubicin (12 mg/m2 daily for 3 days) and adriamycin (1.5 mg/m2 daily for 4 days) followed by radiotherapy with 35 Gy achieved partial remission and 15 months after onset of treatment the patient is in a good general condition.