The aim of this study is to identify insulin-dependent diabetes mellitus (IDDM)-susceptible HLA antigens in IDDM patients who do not have established risk allele, HLA-DQA1*0301, and analyze relationship of these HLA antigens and the degree of beta-cell destruction. In 139 Japanese IDDM patients and 158 normal controls, HLA-A, -C, -B, -DR and -DQ antigens were typed. Serum C-peptide immunoreactivity response (deltaCPR) to a 100-g oral glucose load < or = 0.033 nmol/l was regarded as complete beta-cell destruction. All 14 patients without HLA-DQA1*0301 had HLA-A24, whereas only 35 of 58 (60.3%) normal controls without HLA-DQA1*0301 and only 72 of 125 (57.6%) IDDM patients with HLA-DQA1*0301 had this antigen (Pc = 0.0256 and Pc = 0.0080, respectively). DeltaCPR in IDDM patients with both HLA-DQA1*0301 and HLA-A24 (0.097 +/- 0.163 nmol/L, mean +/- SD, n = 65) were lower than in IDDM patients with HLA-DQA1*0301 only (0.219 +/- 0.237 nmol/L, n = 45, P < 0.0001) and in IDDM patients with HLA-A24 only (0.187 +/- 0.198 nmol/L, n = 14, P = 0.0395). These results indicate that both HLA-DQA1*0301 and HLA-A24 contribute susceptibility to IDDM independently and accelerate beta-cell destruction in an additive manner.