Primary mediastinal B-cell lymphoma (PMBCL) is a rare type of diffuse large B-cell lymphoma, constituting 2-4% of non-Hodgkin lymphomas. Here we present a case of PMBCL initially presenting as pleuritic chest pain in an otherwise healthy 33-year-old woman. PMBCL typically presents as a large, fast-growing tumor limited to the mediastinum, making the iliosacral involvement in this case unusual. R-CHOP is the most commonly used chemotherapy regimen, although more aggressive therapies like EPOCH-R can spare the need for consolidative radiation therapy. PMBCL represents one of the greatest diagnostic and therapeutic challenges in malignant hematology.