Cardiac lymphomas are rare, and the spectrum of pathologic features is not well defined. We encountered an unusual case of cardiac lymphoma residing within a presumed thrombus. To place such cases in context, we reviewed all cardiac lymphomas presenting to a large US cardiovascular medicine referral center during a 30-year period. A total of 14 cardiac lymphomas were identified, and these included 6 primary cardiac lymphomas (PCLs) and 8 lymphomas secondarily involving cardiac structures. Upon review, 3 of the PCLs were confirmed to be diffuse large B-cell lymphoma, not otherwise specified, involving the myocardium. The other 3 cases of PCL lacked myocardial invasion and showed lymphoma cells embedded in fibrin thrombus. Acute inflammation was not evident. These lymphomas presented in immunocompetent male individuals and involved either a prolapsed myxomatous mitral valve, a pseudomyxoma from the left atrium, or a thrombus arising in a synthetic aortic root graft. All 3 consisted of large atypical lymphocytes expressing a nongerminal center B-cell immunophenotype. Two cases were positive for Epstein-Barr virus (latency type III), but none demonstrated human herpes virus-8 latent nuclear antigen. No systemic disease was found at presentation or during follow-up. In our experience, fibrin-associated large B-cell lymphoma arising in the heart represents a substantial proportion of PCL. These lymphomas appear to represent an underrecognized variant of diffuse large B-cell lymphoma with favorable outcome. Further study is needed to understand their natural history.