Centrocytic lymphoma, or mantle cell lymphoma (MCL), is characterized by a chromosomal translocation t( 11;14) (q13;q32) involving the bcl-1 locus on chromosome 11. Cyclin D1 is a cell-cycle regulatory protein essential for G 1 S transition and has been identified as a potential transforming gene affected by the translocation. In this study, 32 cases of MCL were analysed for the bcl-1 rearrangement and cyclin D1 protein expression. In 17 cases, a rearrangement a t the major translocation cluster of bcl-1 could be detected. Twentj-four cases exhibited nuclear cyclin DI expression that was not detectable in other B-cell lymphomas (n=40) or in normal B-cells. In nine MCL samples, cyclin D1 was expressed without a detectable bcl-1 rearrangement. The detection of a t(11;14) by means of classical cytogenetics in one of these cases, however, may suggest that this discrepancy could be due to chromosomal breakages outside the typical translocation cluster region. In two cases, a bcl-1 rearrangement was not accompanied by cyclin D1 expression. This study provides further evidence that cyclin D l is involved in the pathogenesis of M C L and can be exploited as a diagnostic marker in the differential diagnosis of B-cell lymphomas and in the identification of MCL.