IDDM patients of North East Italian region were molecularly typed for their HLA-DQB1 and DQA1 loci by using allele specific oligonucleotide probes and PCR amplified genomic DNA. IDDM status strongly correlated with DQB1 alleles carrying a non-aspartic acid residue in position 57 of DQ beta chain and DQA1 alleles with an arginine residue in position 52 of DQ alpha chain. Genotype analysis revealed that individuals with two DQB1 alleles having a non-aspartic residue in position 57 and two DQA1 alleles with an arginine residue in position 52 had the highest relative risk of disease: they constituted 41% of IDDM patients as compared to 0% of controls. Heterozygosity either at residue 57 of DQB1 or residue 52 of DQA1 was sufficient to abrogate statistical significance for disease association, although 43.6% of IDDM patients were included in these two groups as compared to 21.6% of normal controls. On the other hand the presence of two DQB1 alleles with aspartic acid in position 57 was sufficient to confer resistance to disease irrespective of the DQA1 genotype. Based on the number of possible susceptible heterodimers an individual can form, it was found that 85% of IDDM cases could form two or more heterodimers (two in cis and two in trans), but no IDDM case was found to form one susceptible heterodimer in cis. These results demonstrate that the complete HLA-DQ genotype, more than single DQB1 or DQA1 alleles or DQB1-DQA1 haplotypes, is associated with the highest risk of disease. Screening of the population for preventive purposes and/or early signs of IDDM should then take advantage of this result and "susceptible homozygous" individuals should be followed very closely and considered the first group of choice for possible new therapeutical trials.