Despite extensive study, the pathogenic mechanisms of Behçet's disease remain uncertain. The ocular inflammation caused by this disease is severe, often causing significant visual loss and, although the nature of the cellular infiltrate has been examined in many of the involved organs, the infiltrating cells in inflamed eyes have not. To investigate the mechanisms involved in perpetuating the ocular inflammation, five enucleated eyes from patients with Behçet's disease were examined by immunohistochemical staining using a panel of monoclonal and polyclonal antibodies. Control eyes from patients with chronic intraocular inflammation from other causes were also examined. Cellular infiltrates were a consistent finding in choroid and periretinal scar tissue, formed almost entirely by mononuclear cells. T lymphocytes were found to predominate (largely the CD4+ subset). B lymphocytes and NK cells were infrequent findings but macrophages were present in significant numbers. No complement or immunoglobulin deposits were found. Infiltrating lymphocytes and macrophages were HLA DR positive. Retinal vascular and retinal pigment epithelium were only occasionally positive. Our findings suggest that cell mediated immunity, rather than immune complex deposition is responsible for the perpetuation of the ocular inflammation in Behçet's disease and that CD4+ T lymphocytes play a central role in this.