The PRAD-1/CCND1 gene encodes Cyclin D1, a cyclin involved in cell cycle regulation at the G1-S transition. Over-expression of this gene is a highly specific molecular marker of mantle cell lymphomas (MCLs), but it may also be up-regulated in some chronic lymphoproliferative disorders, mainly chronic lymphocytic leukaemia. We have examined PRAD-1/CCND1 gene expression by Northern blot and Western blot analysis in a series of 18 hairy cell leukaemias (HCLs), nine other splenic malignant lymphoproliferative disorders, and three normal/reactive spleens. Over-expression of the mRNA PRAD-1/CCND1 gene was observed in 16/18 HCLs, including one case of hairy cell leukaemia variant, whereas this molecular alteration was not found in other cases examined. mRNA levels varied from case to case, but they were lower than those observed in MCLs. At the protein level, Western blotting analysis showed Cyclin D1 protein expression in the 11 HCLs analysed. No bcl-1 rearrangements were seen with the MTC, p94PS and PRAD-1 (lambda-P1-4) probes used, and no PRAD-1/CCND1 gene amplification was detected in any case. These findings indicate that PRAD-1/CCND1 is over-expressed at mRNA and protein levels in a high number of HCLs. However, the levels of expression are much lower than in MCLs, and this expression is not associated with bcl-1 rearrangements or PRAD-1/CCND1 gene amplification.