Marrow involvement in 20 patients with non-Hodgkin's lymphoma (NHL) were studied by histology, immunophenotypic and genotypic methods. Eighteen of these trephines were histologically involved with recognizable lymphomatous infiltrates and five of these were the primary disease site. In the remaining two cases (with histologically involved lymph nodes) the trephines were uninvolved with tumour. Three B-cell cases expressing surface immunoglobulin (sIg) and/or CD37 and one case not analysed phenotypically showed Ig gene rearrangements. The two remaining cases with B NHL showed no gene rearrangements, however, in one of these the trephine was histologically uninvolved with tumour. Twelve out of 14 T-cell cases were characterized by variable or absent expression of one or more T-cell antigens from the tumour population, one case was negative for all T-cell antigens and the remaining case was not histologically involved with tumour. All three lymphoblastic lymphomas and only 4/11 peripheral T-cell lymphomas (PTCL) cases revealed T-cell receptor (TcR) gene rearrangements. One of the latter cases also exhibited Ig JH gene rearrangements. This study demonstrates the usefulness of bone marrow trephines (BMT) in histologic, phenotypic and genotypic analyses. However, although genotypic data confirm clonality in B NHL and the lymphoblastic lymphomas there was genotypic heterogeneity within the PTCL group.