The chromosomal aberration t(2:5) resulting in the juxtaposition of NPM and ALK genes is a well-known feature of several Ki-1+ anaplastic large cell lymphomas (ALCL) of the T-cell type. However, conflicting results have been reported concerning the presence of this gene rearrangement in other ALCL and Hodgkin's disease (HD), respectively. We performed NPM/ALK RT-PCR on 14 cases of ALCL expressing distinct myelomonocytic markers, e.g. CD11c, CD13, CD14 or CD68, but neither T-cell nor B-cell associated antigens (null cell phenotype). The specific translocation was found exclusively in six childhood tumours previously diagnosed as malignant histiocytosis (MH), whereas all adult lymphomas (three ALCL without characteristics of MH, three secondary ALCL following HD) and two paediatric cases of secondary ALCL following HD did not show NPM/ALK gene fusion products. By Southern blotting, the status of T-cell receptor (TCR) and immunoglobulin heavy chain genes (IgH) were investigated; two patients with initially diagnosed MH had the TCRdelta-chain gene rearranged (Ddelta2-Ddelta3 and Vdelta1-Jdelta1, respectively). IgH rearrangements were detected in only one patient with secondary ALCL. Our data indicate a high association of previously diagnosed MH and NPM/ALK gene rearrangements. In one case, this specific translocation was demonstrated at an early stage of development; in another, a mature TCRdelta-chain gene rearrangement was detected. These data support the hypothesis of a lymphoid origin of this subgroup of Ki-1 positive ALCL previously diagnosed as MH.