Eighteen patients with the presumed ocular histoplasmosis syndrome were typed for HLA histocompatability antigens and 78% were found to have the B7 antigen. This data implies an underlying immunogenic predisposition to the development of the clinical eye findings and active posterior pole lesions in the presumed ocular histoplasmosis syndrome. It is likely that some feature of the immune system of these patients renders them more susceptible either to the initial infection or to a later activation of posterior pole lesions mediated through immunologic mechanisms.