Bcl-2 is a proto-oncogene inhibiting apoptosis, and p53 is a tumor supressor gene which induces apoptosis. Both seem to take part in tumorigenesis. An inverse relationship between the two genes has been reported in some neoplasms, although the exact mechanism is not fully understood. We have analyzed the expression of bcl-2 and p53 in 18 fetal thymuses and 18 clinically benign and malignant thymomas: bcl-2 was expressed by most medullary lymphocytes and epithelial cells of the normal thymus; p53 was not expressed at all. Bcl-2 and p53 were co-expressed in the majority of the thymomas and the staining reaction was stronger in the clinically malignant ones. It is concluded that although co-expression of bcl-2 and p53 is of doubtful prognostic relevance, the staining pattern of bcl-2 supports the histogenetic classification system of Müller-Hermelink.