In order to investigate the immunogenetic mechanism of Behçet's disease, frequencies of HLA antigens were studied in family members of patients with this disease. The subjects consisted of 37 patients, 74 family members and 49 normal controls. It was shown that the frequencies of HLA-B51 were equally higher in the patients, fathers, mothers and siblings of the patients than in the normal controls. On the other contrary, the frequency of HLA-DQw1 was significantly decreased only in the patients with this disease. These results may suggest that family members of the patients have resistant genetic factors, unlike the patients.