HLA-DR and MT1, MT2, MT3 genotypes have been investigated in 123 Type 1 (insulin-dependent) diabetic subjects and their families. Ninety-eight percent of probands possessed either DR3 (relative risk = 5.0), or DR4 (relative risk = 6.8) or both antigens (relative risk = 14.3), emphasizing the strong association of the disease with these two antigens. Almost 51% of the probands were DR3, DR4 heterozygotes. The DR antigen combinations of the parents leading to DR3, DR4 heterozygous and to DR3 and DR4 homozygous offspring were analysed. There was a marked increase in DR3, DR4 heterozygosity, but no increase in homozygosity for these antigens compared with the expected frequencies. These results are compatible with the existence of two susceptibility genes operating at a locus or at loci closely linked to that of HLA-DR. There was a striking reduction of DR7 (relative risk = 0.1) and only five probands possessed DR2 (relative risk = 0.1). In each case, the other inherited allele was DR3 or DR4. Linkage disequilibrium between B7 and DR2 was much lower in the haplotypes of the probands than in the ‘non-diabetic’ parental haplotype. In contrast, the association of BW62 with DR4 was more pronounced in the haplotypes of the probands. There was no increase in recombination frequency in these families and no strong effect of HLA-DR on age of onset could be demonstrated. There was a significant shift towards DR identity compared with identity for the whole HLA haplotype (A, B, C and DR) in both healthy and diabetic siblings (p < 0.025).