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Diffuse large B-cell lymphomas (DLBCLs) constitute a heterogeneous group of lymphomas in which germinal centre B-cell-like and activated B-cell-like subtypes can be discerned based on pathology, clinical presentation, and gene expression patterns. Testicular DLBCLs form an immune-privileged site-related subgroup of DLBCLs with an unfavourable prognosis. In the present study, cDNA microarray analysis, immunohistochemistry for CD10, Bcl6 and MUM1, and somatic hypermutation analysis of the immunoglobulin heavy chain gene rearrangements were used to determine the subtype of primary testicular DLBCL. Immunohistochemistry revealed 14/22 testicular DLBCLs with an activated B-cell-like immunophenotype and 8/22 with an ambiguous immunophenotype co-expressing CD10 and high levels of MUM1. cDNA microarray analysis of these 22 and four additional cases showed a uniform activated B-cell-like gene expression pattern for both immunophenotypes. Somatic hypermutation analysis of immunoglobulin heavy chain genes showed a very high mutation load in seven cases tested, but intraclonal heterogeneity was found at low level in only one of these cases. It is concluded that primary testicular DLBCLs have uniform activated B-cell-like subtype characteristics despite a number of cases showing an ambiguous immunophenotype. Copyright © 2006 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.