Genetic factors appear to be important in the pathogenesis of Behçet's disease. Although it is known to be strongly associated with HLA-B 51, the association of HLA class I antigens with specific clinical findings of the disease has not been studied extensively and the few studies are conflicting. The aim of this study was to investigate the association of HLA class I alleles with the manifestations of Behçet's disease in Turkish patients. Eighty-five patients with Behçet's disease were typed for HLA-A, B, and C antigens with the serologic, standard microlymphocytotoxicity technique. Possible associations of the HLA complex with clinical findings of Behçet's disease were examined. Statistically significant findings are as follows (P < 0.05): increased HLA-B 51 and decreased HLA-B35 frequency in patients with thrombophlebitis, increased HLA-A29 and decreased HLA-Bw6 frequency in patients with ocular involvement, decreased HLA-Cw2 frequency in patients with erythema nodosum, and decreased HLA-Cw 7 frequency in patients with genital ulceration. Of particular note, the results of this study suggest that the presence of HLA-B 51 and the absence of HLA-B35 can be regarded as laboratory risk factors of venous thrombosis in patients with Behçet's disease.