Mcl-1 protein is a new member of the bcl-2 protein family. It is believed to be a blocker of apoptosis but might be different from bcl-2 in the control of apoptosis. Using immunostaining of formalin-fixed, paraffin-embedded sections, we investigated the expression of Mcl-1 in 42 thymic epithelial tumours: three medullary thymomas, five mixed thymomas, seven cortical thymomas, eight well-differentiated thymic carcinomas, 14 squamous cell carcinomas, four lymphoepithelioma-like carcinomas and one undifferentiated carcinoma, bcl-2 immunocytochemical localization was also performed for comparison. High-grade thymic carcinomas, especially squamous cell carcinomas, revealed more intense Mcl-1 immunoreactivity as compared to other subtypes (P < 0.001). In cases that co-expressed Mcl-1 and bcl-2, the less differentiated cells had more intense expression of bcl-2, while the more differentiated cells displayed stronger Mcl-1 immunoreactivity. The differential expression of Mcl-1 and bcl-2 in neoplastic cells provides evidence that these proteins may play different roles in the processes of programmed cell death in thymic neoplasms. The finding that thymic carcinomas have stronger immunoreactivity for Mcl-1 indicates that this protein could be a useful marker to differentiate aggressive thymic epithelial tumours from indolent ones.