T-cell rich B-cell non-Hodgkin's lymphoma (T-cell rich B-cell lymphoma) is a morphological variant of diffuse large B-cell lymphoma. It is important to recognize this variant in the differential diagnosis of T-cell non-Hodgkin's lymphoma. The main differential diagnosis of T-cell rich B-cell lymphoma, nodular and diffuse lymphocyte predominance Hodgkin's disease (lymphocyte predominance Hodgkin's disease), is, however, even more difficult and differentiating criteria are still not satisfactorily defined. Moreover, T-cell rich B-cell lymphoma may not represent a clinicopathological entity. Twelve cases of T-cell rich B-cell lymphoma, selected on the basis of morphology and limited immunohistochemistry without previous knowledge of clinical data, were studied by immunohistochemistry and polymerase chain reaction for bcl-2 rearrangements to investigate the histogenetic background. In three of 12 cases, bcl-2 rearrangements were found, strongly suggesting a follicle centre cell origin. In three other cases, a documented history of definite nodular lymphocyte predominance Hodgkin's disease 29 months to 20 years prior to the diagnosis of the lymphoma was present. No differences in growth pattern, residual nodularity, tumour cell distribution, cellular morphology and composition, or immunophenotypical differences were noted in these six cases as compared to the remaining cases. These data underscore the histogenetic diversity in T-cell rich B-cell lymphoma and identify it as a progressed form of lymphoma derived from entities as diverse as follicle centre cell lymphoma and nodular lymphocyte predominance Hodgkin's disease. Moreover, it shows a complete morphological overlap with the diffuse form of lymphocyte predominance Hodgkin's disease and may actually encompass this disease entity.