The Basques were previously shown to present a high frequency of HLA-B18 and BfF1, which are known to be associated with insulin dependent diabetes mellitus (IDDM). During the VIII International Histocompatibility Workshop, we studied HLA-A, B, C, DR; Bf, C4 and GLO.I polymorphisms in 51 unrelated French Basque IDDM patients and in 50 controls. Haplotypes were established by family studies in all controls and some patients. Two haplotypes were frequently found in the controls: HLA-A1, Bw57, BfS, C4 F1S, DR7 and HLA-Aw30, Cw5, B18, Bf F1, C4Fs degree, DR3. The first one was not found in the patients. All the components of the second haplotype had increased frequencies possibly as a consequence of linkage disequilibrium with HLA-DR3: a highly significant association between IDDM and HLA-DR3 was observed (90.2% vs 24.0%, relative risk (RR) = 29.1, P less than 10(-11)). The HLA-DR4 frequency was slightly increased (37.3% vs 16.0%), and HLA-DR2 was not found. The silent allele C4s degree was particularly associated with early diagnosed IDDM (86.7% in patients with age at onset under 20 years vs 57.1% in other patients, P less than 0.02). The high relative risk for HLA-DR3/DR4 heterozygous vs that of individuals, possibly HLA-DR3 homozygous, supported the hypothesis that two HLA-DR linked genetic factors could be involved in the inheritance of IDDM susceptibility.