In a previous fluorescent in situ hybridization (FISH) study of patients with high-grade follicle centre lymphomas (FLCs), we often found additional copies of chromosome 7 in bone marrow (BM) cell nuclei even though obvious malignant tumour cells could not always be morphologically identified in the corresponding cell smears. This raised the question whether the gains of chromosome 7 are really confined to B-lymphoid tumour cells or whether other cell lineages are also of clonal origin. In the present investigation we employed FISH in combination with immunomarkers and morphological studies on BM smears and lymph node imprints from seven patients with high-grade FCLs and diffuse large B-cell lymphomas (DLBCLs). Three out of seven BM samples were found to contain clonal CD20-positive B-lymphoid cells (range 0.4-96% of the cells) and no extra copies of chromosome 7 were detected in the myelomonocytoid or erythroid cells or in the CD3-positive T lymphocytes. All seven patients showed additional copies of chromosome 7 in the lymph nodes and, again, this cytogenetic abnormality was also restricted to the CD20-positive cells (range 0.7-80% of the cells). Thus the present findings confirm that high-grade B-cell lymphomas with or without BM engagement involve the CD20-positive B-lymphoid cells exclusively and not the T lymphocytes, erythroid or myelomonocytoid cell lineages. These findings may indicate that anti-CD20 immunotherapy could be of value in high-grade B-cell lymphomas.