PURPOSE Malignant transformation of cells is frequently associated with abnormalities in HLA expression. These abnormalities may play a significant role in the clinical course of the disease because the cellular immune response to tumors relies on concomitant recognition of tumor antigens with self-HLA molecules. Since HLA molecules mediate interactions of tumor cells with specific receptors on T and natural killer (NK) cells, tight control of expression of HLA molecules is critical for initiation and implementation of an effective cellular immune response. There is not much information on HLA class I antigens in conjunctival carcinomas. We studied the immunoexpression of HLA class I antigen and beta2-microglobulin (beta2-m) in conjunctival dysplasias and conjunctival squamous cell carcinomas. METHODS HLA class I antigen and beta2-m expression were analyzed in 10 conjunctival dysplasias, 6 carcinomas in situ, and in 11 conjunctival carcinomas by immunoperoxidase staining with monoclonal antibodies to HLA class I antigen and beta2-m on the archival clinical samples. Immunoanalysis was done according to the International Histocompatibility Working Group Project Description. RESULTS HLA class I antigen and beta2-m were heterogeneous in 10 conjunctival dysplasias, negative in 6 carcinomas in situ, and 11 conjunctival squamous cell carcinomas. The difference of immunoexpression for HLA class I and beta2-m among the 3 groups was statistically significant (P <0.001). CONCLUSIONS HLA class I antigens and beta2-m are decreased in conjunctival dysplasias and negative in carcinomas in situ, and carcinomas. The findings support the role of cytotoxic T lymphocyte-mediated control of tumor growth in the clinical course of conjunctival squamous cell carcinomas.