Frequency of HLA-A, B, and C antigens was studied in 184 patients with Behçet's disease to investigate the immunogenetically determined predisposition to this disease. A statistically significant increased incidence of HLA-B5 or Bw51 was noted in the patients as compared with the normal control subjects. No significant difference was observed in the distribution of other HLA antigens. On the basis of HLA studies, it seems likely that the susceptibility genes to Behçet's disease closely linked to HLA-Bw51 may have been spread by the old nomadic tribes or the Turks via the Silk Route.