Primary mediastinal large B-cell lymphoma (MLBCL) shares important clinical and molecular features with classic Hodgkin lymphoma, including nuclear localization of the nuclear factor kappaB (NFkappaB) subunit c-REL (reticuloendotheliosis viral oncogene homolog) in a pilot series. Herein, we analyzed c-REL subcellular localization in additional primary MLBCLs and characterized NFkappaB activity and function in a MLBCL cell line. The new primary MLBCLs had prominent c-REL nuclear staining, and the MLBCL cell line exhibited high levels of NFkappaB binding activity. MLBCL cells expressing a superrepressor form of inhibitor of kappa B alpha signaling (IkappaB alpha) had a markedly higher rate of apoptosis, implicating constitutive NFkappaB activity in MLBCL cell survival. The transcriptional profiles of newly diagnosed primary MLBCLs and diffuse large B-cell lymphomas (DLBCLs) were then used to characterize the NFkappaB target gene signatures of MLBCL and specific DLBCL subtypes. MLBCLs expressed increased levels of NFkappaB targets that promote cell survival and favor antiapoptotic tumor necrosis factor alpha (TNFalpha) signaling. In contrast, activated B cell (ABC)-like DLBCLs had a more restricted, potentially developmentally regulated, NFkappaB target gene signature. Of interest, the newly characterized host response DLBCL subtype had a robust NFkappaB target gene signature that partially overlapped that of primary MLBCL. In this large series of primary MLBCLs and DLBCLs, NFkappaB activation was not associated with amplification of the cREL locus, suggesting alternative pathogenetic mechanisms.