A female patient in her mid-60s presented with progressive shortness of breath, pleuritic chest pain and bilateral leg swelling for 1 week. Initial diagnostic workup revealed pericardial effusion, and a localised pericardial tubular mass on CT chest. Pericardial fluid analysis showed elevated white cells, with predominance of medium-large sized atypical lymphoid cells. Atypical lymphocytes stained positive for CD79a, CD10, PAX-5, BCL-2 and BCL6. Fluorescence in situ hybridisation testing demonstrated MYC and BCL6 rearrangements without BCL2 gene rearrangement. The overall morphological, immunohistochemical and cytogenetic findings supported a diagnosis of high-grade B cell lymphoma with MYC and BCL6 rearrangements. After extensive staging workup, localised disease involving the pericardium with a diagnosis of primary cardiac large B cell lymphoma was established. She was treated with dose-adjusted etoposide, prednisone, vincristine, cyclophosphamide, doxorubicin and rituximab chemotherapy. Rituximab was discontinued owing to largely absent CD20 expression. Interim positron emission tomography-CT after three cycles revealed a complete response, and the patient completed six cycles of therapy.