Mantle cell lymphoma (MCL) is a distinct type of B‐cell non‐Hodgkin's lymphoma characterized by the t(11;14)(q13;q32) and cyclin D1 overexpression. Defects in apoptosis may contribute to pathogenesis. This study evaluated the expression of the anti‐apoptotic protein Mcl‐1 in two MCL cell lines and five frozen MCL tumours (four small‐cell, one blastoid/large‐cell) using western blot analysis. Mcl‐1 expression was also assessed in 36 formalin‐fixed, paraffin wax‐embedded MCL tumours (24 small‐cell, 12 blastoid/large‐cell) by immunohistochemistry. Western blot analysis revealed the expected 37 kD protein product in both MCL cell lines and in five frozen tumours, with the blastoid case having the highest expression level. Using a cut‐off of >10% immunolabelled cells for Mcl‐1, it was found that 12 of 36 MCL tumours were positive. Mcl‐1‐positive tumours had a higher frequency of blastoid/large‐cell morphology (8/12 versus 4/24, p = 0.009), p53 overexpression (3/10 versus 1/23, p = 0.04), and higher Ki67 immuno‐labelling (p = 0.002). It is concluded that expression of Mcl‐1 in MCL is heterogeneous. A relatively high level of Mcl‐1 expression correlates with high‐grade morphology, a high proliferative state, and p53 overexpression. Copyright © 2002 John Wiley & Sons, Ltd.