Cardiac involvement in non-Hodgkin's lymphoma is a rare occurrence with a dismal prognosis, which may evolve with different clinical presentations, the most frequent being heart failure. Diagnosis of cardiac involvement is generally made by cardiac ultrasound. We report a case of lymphomatous pericarditis in the evolution of a non-Hodgkin's lymphoma, diagnosed by PET-CT scan, and occurring concomitantly with complete isotopic remission of enlarged mediastinal lymph nodes following chemotherapy.