Classification and differential diagnosis of non-Hodgkin lymphomas involving the serous body cavities and presenting as effusion lymphomas. Lymphomas primarily involving the serous body cavities include a certain number of BLs, mainly occurring in the context of AIDS, which present as primary lymphomatous effusions without mass formation. The most specific biologic markers discriminating PEL from BL presenting as a primary lymphomatous effusion are represented by KSHV infection (assessed by ORF73/LNA-1 immunoreactivity), which clusters with PEL, and by translocation of the c-MYC proto-oncogene, which segregates with BL. KSHV-unrelated large B-cell lymphomas, also termed as KSHV-unrelated PEL-like lymphomas, can be differentiated from PEL because the neoplastic cells do not display evidence of KSHV infection, but display features related to large B-cell lymphoma. KSHV/HHV8 indicates Kaposi sarcoma herpesvirus/human herpesvirus 8; EBV, Epstein-Barr virus; IBL, immunoblastic lymphoma; ALCL, anaplastic large cell lymphoma; DLBCL, diffuse large B-cell lymphoma; BL, Burkitt lymphoma; and PAL, pyothorax-associated lymphoma, now called DLBCL with chronic inflammation.