Selective cardiac involvement by a malignant non-Hodgkin's lymphoma (primary lymphoma of the heart) is extremely rare. Cardiac lesions occurring in the course of a malignant non-Hodgkin's lymphoma (secondary lymphoma of the heart) are found at autopsy in about 10 per cent of the cases. Secondary lymphoma of the heart may involve all cardiac structures, but lesions of the right heart, and particularly the right atrium, are predominant. Clinical manifestations are usually non-specific and appear at a late stage, reflecting diffuse involvement. When the malignant non-Hodgkin's lymphoma is known, systematic two-dimensional echocardiography should easily detect the lymphomatous cardiac infiltration, irrespective of the presence or absence of symptoms. When the malignant non-Hodgkin's lymphoma is unknown, only histology can show that the heart tumour is a lymphoma, evaluate its extension and confirm that it is limited to the cardiac walls, provided the disease is treated at an early stage, prolonged complete remissions are observed after chemotherapy and/or radiotherapy.