The HLA-BS phenotype was investigated in 235 patients with Behcet's disease and in 100 healthy persons. HLA-BS was more frequent in the patients (77 vs. 30% p<0.001, relative risk=7.8). The frequency of HLA-BS was higher in patients with genital ulceration than without (82.3 vs. 63%, p<0.01). Patients with thrombophlebitis showed a less frequent positivity of HLA-BS antigen than the patients without thrombophlebitis (50 vs. 79.2%, p<0.02). These results suggest that HLA-BS-related genes not only affect the development of Behcet's disease but also the occurrence of its clinical manifestations.