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The diffuse aggressive B-cell lymphomas, as recognised in the 2001 WHO classification, represent a clinically and biologically heterogeneous group of neoplasms that require very different therapeutic approaches and have very different outcomes. They should be diagnosed using a multiparameter approach that emphasises morphological and immunophenotypic studies, and in at least some cases, relies on cytogenetic and/or genotypic studies. Incorporation of clinical data may be important as well. There is also current interest in going beyond the basic diagnosis and providing pathological prognostic information when possible. Whereas the diagnosis of some cases will be relatively easy, the differential diagnosis in others is very difficult, with some cases in a grey zone between two different well defined categories. See end of article for authors ’ affiliation