Primary cardiac lymphoma, which is very rare, is generally regarded to have a poor prognosis. A case of a 69-year-old man with primary cardiac lymphoma diagnosed by antemortem examination is reported. A computed tomography scan of the chest demonstrated a huge right atrial mass with invasion into the other chambers. No mediastinal lymphadenopathy was detected. Cytologic analysis of pericardial effusion revealed diffuse large B-cell type non-Hodgkin malignant lymphoma. The patient died on the 18th day of chemotherapy (cyclophosphamide, hydroxydaunomycin, oncovin, and prednisone) due to low-output syndrome and multiple organ failure. At autopsy, massive gray-white tumor almost occupied the right atrium and invaded the right inferior lobe of the lung. Although prognosis of primary cardiac lymphoma remains poor, early diagnosis may improve the prognosis.