The purpose of our study was to investigate the role of immune mechanisms in the pathogenesis of pterygium using an immunohistochemical technique. Our material consisted of 35 surgically excised pterygia and 7 samples of normal conjunctiva obtained from an equal number of patients. HLA-DR antigen expression in epithelial cells, B-cells, suppressor and helper lymphocytes, Langerhans' cells, and monocytes/macrophages were studied immunohistochemically in frozen sections using anti-human HLA-DR, anti-CD22, anti-CD8, anti-CD4, anti-CD1a, and anti-LeuM5 monoclonal antibodies. Aberrant HLA-DR antigen expression in epithelial cells was detected in 30 of 35 cases of pterygium. Epithelial cells in samples of normal conjunctiva were found to be negative in HLA-DR antigen expression. HLA-DR antigen expression in pterygium was found to be closely related to the density of T4 cells and, especially, of CD4 lymphocytes. The present findings suggest that an immunopathologic mechanism plays a role in the pathogenesis of pterygium.