Primary cardiac lymphoma (PCL) is a rare and extremely aggressive malignancy, which can express itself by damaging the cardiac conduction system (complete atrio-ventricular block), the myocardium and the pericardium. Histology confirms the diagnosis. Chances of survival depend on early diagnosis and treatment. We present the case of a female patient admitted for severe deterioration of her general state of health who had a complete atrio-ventricular block caused by a tumour of the atrial septum, a B-cell lymphoma stage I. The diagnosis was suggested by combined PET/CT imaging and confirmed by histology of a cardiac biopsy. Her general clinical condition, including the cardiac parameters, significantly improved following chemotherapy induction.