Demonstration of cyclin D1 expression by immunohistochemistry in a CD5-positive small B-cell proliferation is extremely helpful in the diagnosis of mantle cell lymphoma in tissue samples, including bone marrow trephine biopsies (BMTB) and in differentiating them from chronic lymphocytic leukaemia (CLL). Following the identification of cyclin D1 expression in one case of CLL on BMTB, 64 additional cases, which included 25 lymph nodes, one tonsillar lesion, one skin lesion and 37 BMTBs were systematically reviewed for presence of cyclin D1 overexpression. Overall, in seven of 65 samples (approximately 10%) of CLL, a minority of the leukaemic cells in the proliferation centres expressed cyclin D1. Cytogenetic analysis had been performed in three of seven cases and there was no evidence of translocation involving CCND1 locus. Our findings suggest that a small subset of CLL overexpresses cyclin D1 in amounts that can be demonstrated by immunohistochemistry. Our observation has impact on the diagnosis of small B-cell lymphomas in BMTB and other tissue samples.