Mediastinal B-cell lymphoma (MBL) is a distinct variant of aggressive non-Hodgkin's lymphoma with characteristic clinical and biological features but less well-defined histomorphology. We reevaluated 124 biopsy specimens from 109 MBL patients of an Italian/French/German retrospective clinical study. MBL was primarily diagnosed on clinical and histological grounds in conjunction with the detection of CD20 expression by immunohistology. Cytologically, MBL features limited intralesional but considerable interindividual cytological diversity, ranging from medium-sized to very large, atypical cells. Sclerosis and necrosis are restricted to extrathymic and extranodal sites of involvement, predominantly the lung, as is angioinvasion, which predominantly affects larger vessels. The medium-sized and the large cell variants resemble marginal zone lymphoma variants, whereas the very large cell variant of MBL has not so far been found to have any extramediastinal counterpart. We conclude that MBL displays a broad morphological spectrum covering more than is implied by the term "diffuse large cell lymphoma." Because statistical analysis of cytological and histological criteria failed to correlate with prognosis in this comprehensive group of patients, we think it inadvisable further to subclassify MBL.