A 54-year-old HIV + man with multicentric Castleman disease on rituximab presented with altered mental status secondary to a right thalamic mass on computed tomography (CT) scan (panel A, CT scan). A biopsy of the mass revealed medium-sized pleomorphic lymphoma cells (panels B-C; hematoxylin-and-eosin stain; panel B, original magnification ×100 [10× objective]; panel C, original magnification ×400 [40× objective]). Immunohistochemically, the lymphoma cells were positive for OCT2 (panel D, original magnification ×400 [40× objective]), CD3 (panel E, original magnification ×200 [20× objective]), MUM-1 (panel F, original magnification ×200 [20× objective]), and human herpesvirus 8 (HHV-8; panel G, original magnification ×200 [20× objective]). CD45 was weakly positive (panel H, original magnification ×400 [40× objective]). CD30 and CD138 were positive in a few cells. In situ hybridization for Epstein-Barr virus–encoded RNA was positive (panel I, original magnification ×200 [20× objective]). Stains for CD20, CD79a, PAX-5, and EMA were negative. Ki-67 revealed a high proliferation rate of >90% (panel J, original magnification ×200 [20× objective]). The diagnosis was extracavitary variant of primary effusion lymphoma (PEL) with aberrant CD3 expression. The patient was started on methotrexate and radiation.PEL is a large B-cell neoplasm, related to HHV-8 infection, usually seen in immunocompromised patients, presenting as serous effusion. An extracavitary variant presenting as a solid mass in the absence of an effusion has been described and can be challenging to diagnose. The prognosis is usually poor.