Malignant non-Hodgkin's lymphomas are rare in the absence of human immunodeficiency virus infection and it is exceptional for a cardiac site to be the prominent feature. In our case, the malignant lymphoma was revealed by pericardial effusion in a context of alteration of the general state. Echocardiography revealed a heterogeneous mass in the right atrium and an abundant circumferential pericardial effusion. Thoracic computed tomography allowed local staging and magnetic resonance imaging (MRI) allowed a better definition, than CT scan, of the extension of the tumour into the various cardiac structures. The histological diagnosis was established on biopsy of a mediastinal lymph node. The patient died 7 months after the diagnosis, despite chemotherapy. The authors emphasize the contribution of echocardiography in the diagnosis of cardiac tumours, computed tomography in local staging, and MRI in the analysis of the various cardiac structures.