In mantle cell lymphoma (MCL) a recurrent chromosomal rearrangement, t(11;14)(q13;q32), has been described. Most breakpoints have been detected within the 120 kb BCL-1 region, upstream of the cyclin D1 gene. To evaluate the association between BCL-1 rearrangement and expression of cyclin D1 in lymphoproliferative disorders, we analysed a series of 24 MCL, 56 other B-cell non-Hodgkin's lymphomas (NHL), 28 chronic B-cell leukemias, 18 hematopoietic cell lines and 10 normal lymphoid tissues at the RNA level. Hematopoietic cell lines with a known 11q13 translocation showed high expression of the 4.5 kb cyclin D1 transcript. Three B-cell lines without known 11q13 breakpoint showed low expression. We detected high expression in all (11/11) MCL with and in 11 out of 13 cases of MCL without detectable t(11;14) rearrangement. In three cases with a rearrangement at the 3' end of cyclin D1, two showed overexpression of the 1.5 kb transcript and one expression of an aberrant (3.0 kb) transcript. In other lymphoproliferative disorders, only 5/15 hairy cell leukemias, all without detectable t(11;14), and 5/8 B-cell leukemias suspected to be MCL in leukemic phase showed expression levels comparable to MCL, whereas no or only low expression were observed in 56 cases of other NHL, seven chronic B-cell leukemias and all (10/10) normal lymphoid tissues. Cell sorting experiments on fresh tonsils showed that this low expression was present in normal B-cells and not in T-cells. In contrast to other studies, our data indicate that cyclin D1 is expressed in many lymphoproliferative disorders and normal tissues, albeit at low levels. High levels of expression of cyclin D1 however is restricted to MCL and some hairy cell leukemias. We therefore propose that overexpression of cyclin D1 is a reliable marker for the classification of MCL.