Lymphoma is a classical complication of cardiac transplantation. The authors report a case in which the disease affected the cardiac graft. There were few symptoms and a good response to treatment was obtained. The patient underwent cardiac transplantation at 48 years of age because of primary dilated cardiomyopathy. Four years later, she presented with general ill health and a left supraclavicular lymphadenopathy which led to the diagnosis of a tumour of the atria extending into the mediastinum. MRI was the investigation which gave the best images of tumour extension. As with most late lymphomas of transplanted patients, the proliferation was a monoclonal B type. A significant reduction of tumour size was observed during the 13 months follow-up with chemotherapy and there were no cardiac functional signs.