CD30 expression is found on Hodgkin and Reed-Sternberg cells, anaplastic large cell lymphoma cells and on activated B or T lymphocytes. Recently CD30 was shown to be a transmembrane receptor that is significantly homologous to the tumor necrosis factor receptor (TNFR) family. Ligands for most members of this family, including CD30, have now been identified. This review summarizes the role of the different TNFR family members in lymphocyte proliferation and differentiation in an attempt to understand more clearly the role of CD30 expression in the pathogenesis and clinical behavior of non-Hodgkin's lymphomas. We state that CD30 expression is of prognostic relevance in primary cutaneous and nodal T cell lymphomas in contrast to the absence of clinical relevance of CD30 expression in B cell lymphomas.