The distribution of HLA-DR antigens was investigated in 38 Mexican Mestizo patients with Guillain-Barré syndrome (GBS) and in 100 healthy controls belonging to the same population. IgG, IgM, IgA, CH50, C3, C4 and the number of T and B lymphocytes were also evaluated in the patients. Only DR3 was significantly increased in the patients (Yates' chi 2 = 9.943, Pc = 0.014) and the relative risk for developing the disease was 3.49. These findings support the hypothesis that DR3 or a closely linked Ir gene may play some role in the susceptibility to GBS.