Purpose: The etiopathogenesis of pterygia is controversial. We wished to explore the effects of inflammatory cells and mediators such as T-lymphocyte subpopulations (CD4 and CD8), mast cells, intercellular adhesion molecule-I (ICAM-I), and HLA-DR receptors on pterygium development. Methods: Immunohistochemical stainings were performed for T-lymphocyte subpopulations, mast cells, ICAM-I, and HLA-DR on sections of pterygium specimens. Nasal bulbar conjunctival specimens of patients undergoing cataract or vitrectomy operations were used for comparision. The mean number of mast cells/mm(2) was calculated using an ocular grid, and the difference between the numbers derived from pterygium and from normal conjunctival sections was analyzed statistically using the Mann-Whitney U test. CD4 and CD8 lymphocytes in the epithelial and subepithelial regions, and ICAM-I and HLA-DR expression on pterygium epithelium were compared qualitatively. Results: Increased CD4 and CD8 lymphocytic infiltration in pterygium sections was observed. Only occasional lymphocytes were detected in normal conjunctival sections. The CD4/CD8 ratio was 0.33 for pterygium epithelium and I.34 for pterygium substantia propria. Increased HLA-DR and scattered ICAM-I expression were also detected on pterygium epithelium, but not on the normal conjunctival epithelium. The difference in mast cell numbers between the pterygium and control groups was not statistically significant. Conclusions: The detection of T-lymphocyte infiltration and ICAM-I and HLA-DR expression in pterygium epithelium strongly supports the suggestion that cellular immunity plays an important role in pterygium formation. Pterygium epithelium also seems to participate actively in the augmentation of this inflammatory process by expressing ICAM-I and HLA-DR molecules. Mast cells may participate in some stages or subtypes of pterygium during its development.