Primary cardiac non-Hodgkin lymphomas are fast-growing intracavitary and/or intramyocardial nodular masses, while secondary lymphomas most commonly infiltrate the cardiac tissue. By any definition, cardiac non-Hodgkin lymphomas usually manifest through arrhythmias, refractory heart failure, pericardial effusion, and embolic stroke. We here describe a case of a cardiac non-Hodgkin lymphoma in which the following, previously undescribed features manifest simultaneously. It occurred in a polytransfused hepatitis C virus-positive splenectomized thalassemic patient; it rapidly grew, giving rise to an enormous right atrial mass and, this notwithstanding, it was completely asymptomatic. This cardiac lymphoma was discovered during staging for a CD20+ large B-cell lymphoma of the tonsils. In particular, transesophageal echocardiography, showing that this prolapsing mass had a wide base on the atrial wall, led us to strongly suspect the lymphomatous origin of the mass itself. Notwithstanding anti-CD20 antibody therapy, urgent surgery was unavoidable and histology revealed that the mass consisted of lymphoma proliferation infiltrating even the right atrial wall and the pericardium. During the postoperative course the patient presented with a massive, fatal hemopericardium consequent to intravascular disseminated coagulation. This very unusual case, occurring in a hepatitis C virus-positive thalassemic patient, suggests that a case control study on the incidence of non-Hodgkin lymphoma in such patients may be interesting.