The possible involvement of p53 tumor suppressor gene in the pathogenesis of Hodgkin's disease (HD) is suggested by the frequent finding of abnormal accumulation of p53 protein in the nuclei of Reed- Sternberg cells and their variants (H-RS) in a large proportion of cases. This finding, besides being consistent with the presence of p53 gene mutations, might represent a consequence of the inactivating interaction between p53 and p53-binding proteins such as the product of the MDM2 cellular oncogene. We have examined an unselected series of 77 HD cases of different histologic patterns for the expression of p53 and MDM2 proteins, using specific monoclonal antibodies and sensitive immunohistochemical techniques in single- and double-marker combination. In the large majority of cases (66/77), a variable proportion of H-RS cells expressed MDM2 that was strictly confined to the nuclei. Coexpression of both MDM2 and p53 was common in the same cells. The abnormal nuclear expression of p53 and MDM2 did not seem to correlate with the presence of Epstein-Barr virus infection, as shown by the results of LMP-1 antigen expression and EBER in situ hybridization analysis. Our data suggest that the abnormal accumulation of MDM2 and p53 proteins in HD might reflect a derangement of molecular mechanisms that could play a pathogenetic role in this disease.