To determine the association of histocompatability antigens (HLA) with insulin-dependent diabetes (IDD) in Mexican-Americans, we determined HLA-A, -B, and -C specificities in 112 unrelated patients and 332 controls, and HLA-DR specificities in 85 patients and 209 controls. We also studied immunoglobulin G (IgG) insulin antibody formation in 56 Mexican-Americans with IDD, and the relationship between antibody formation and HLA-DR antigens. IDD patients have a significant increase in HLA-DR4 compared to the control population (chi 2 = 14.75; corrected P less than 0.0001). HLA-DR2 was not detected in any patient with IDD. A significant association between HLA-Aw30 and HLA-B18 was found in IDD patients (chi 2 = 9.39; P less than 0.05) as compared to controls. IgG insulin antibody formation was significantly increased in HLA-DR3- and -DR4-negative patients compared to that in patients positive for both antigens (P less than 0.05). These findings support previous observations in caucasians and black Americans indicating that HLA-DR specificities are associated with IDD and may play a role in determining its mode of inheritance, and perhaps its pathogenesis, independent of ethnic differences. HLA-DR immune-associated antigens are also of importance in determining IgG insulin antibody formation.