We report the case of a 48-year-old man with a clinical syndrome caused by both pulmonary and systemic venous congestion. Echocardiography indicated a huge tumor occupying both dilated atria almost completely, seeming to invade from the atrial septum. A test for HIV was negative. Our primary diagnosis was a biatrial myxoma. Urgent surgery was performed disclosing large cell B-cell primary cardiac lymphoma. The patient died 8 days later. Primary cardiac lymphoma is extremely rare in immunocompetent patients. Its rarity and heterogenous clinical presentation make diagnosis difficult. As tissue characterization is still beyond the capabilities of currently available echocardiography, a combination of examinations are indicated, when the clinical condition is relatively stable, in order to reach the exact diagnosis.