HLA B5 plays an important role in the aetiology of Behcet's disease. The frequency of HLA B5 differs between racial groups and geographical regions. Ocular involvement, which could lead to serious complications, is a very important feature of Behcet's disease. Various racial groups having different genetic properties live in the Cukurova region, located in southern Turkey. To investigate the role of HLA B5 in Behcet's disease in Cukurova region, HLA B5 antigens were investigated by lymphocytotoxicity assay in 65 patients and 200 healthy controls. A significantly higher frequency was found in the patient group. Neither ocular involvement, nor the type of uveitis showed any correlation with HLA B5, but it was found that patients presenting with frequent ocular attacks had significantly higher HLA B5 positivity than the patients with rare attacks. This observation suggests that HLA B5 would be a negative factor in visual prognosis in Behcet's disease.