An immunohistochemical study was conducted on 29 cases of sympathetic ophthalmia (SO). Monoclonal antibodies against T, B, NK cells, macrophages, and MHC class II antigen (HLA-DR) were used. The choroidal infiltrate in 20 eyes was predominantly T cell while B cell predominated in four cases. All eyes with a B cell predominance came from males. A predominance of B cells was correlated to a longer duration of the disease (> 9 months) and in eyes showing phthisical changes. There was no correlation between a predominance of B cells with age, race, corticosteroid treatment or histological type (typical or atypical). These findings suggest that, although SO is a T cell mediated disease, the predominance of B cells in some cases may represent the end stage of the disease process, or seems a secondary pathological process. The kinetic change in cell populations during the disease may have therapeutic implication.