Primary or secondary cardiac lymphomas are not frequent. Their clinical expression is unusual and the diagnosis is rarely made during the patient's life. Our case report, which is a slow atrial flutter with a pericardial effusion, is an uncommon discovery mode for a malignant lymphoma. Their diagnosis and the mechanism of the arythmia were allowed by non-invasive cardiac imagery (transesophageal echography and magnetic resonance imaging), which showed a tumour-like infiltration of the right atrium, of the right ventricle posterior wall, and of the atrioventricular junction. The diagnosis of a high grade B cell malignant non-hodgkin lymphoma, involving the bone marrow, the liver and the kidneys was made by biopsies of lymph nodes, histological analysis of the bone marrow, and a body CT scan. Throughout the first chemotherapy sequence, we observed a spontaneous return to a sinusal rhythm, and the cardiac MRI showed a regression of the myocardial infiltration and of the pericardial effusion; moreover, the patient's state improved and the peripheral lymph nodes shrank back to a normal size. However, the patient passed away, due to neurological complications 13 months after the diagnosis of lymphoma, without recurrence of cardiac involvement.