The HLA-B5 phenotype was investigated in 235 patients with Behçet’s disease and in 100 healthy persons. HLA-B5 was more frequent in the patients (77 vs. 30% p < 0.001, relative risk = 7.8). The frequency of HLA-B5 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-B5 antigen than the patients without thrombophlebitis (50 vs. 79.2%. p < 0.02). These results suggest that HLA-B5-related genes not only affect the development of Behçet’s disease but also the occurrence of its clinical manifesta