Type 1 (insulin-dependent) diabetes mellitus is associated with specific high-risk HLA DQ and DR haplotypes and islet cell antibodies. IDDM susceptibility in Caucasians is more strongly associated with DQ2/DQ8 (DQA1*0501-DQB1*0201/DQA1*0301-DQB1*0302) and DQ6 (B1*0604) than with DRB1*03/DRB1*04, while a single copy of DQ6 (B1*0602) gives sufficient protection against type 1 diabetes. As a part of the ABIS (All Babies in Southeast Sweden) study we have done typing of DQA1, DQB1, and DRB1 by polymerase chain reaction (PCR) amplification of the second exon of the genes, manually dot-blotting onto nylon membranes synthetic sequence-specific oligonucleotide (SSO) probes, 3' end-labeling with (32)P-dCTP, and hybridization followed by stringency washes and autoradiography using the SSO probe. Among 3756 newborns born in southeast Sweden we have found the high-risk genotype DQ2/DR3-DO8/DR4 to be present in 1%, haplotype DQ8/DR4 in 7.8%, and haplotype DQ2/DR3 in 9.6%. DQ2/DR3 or DQ8/DR4 was carried by 16.4% of newborns; the low-risk DQ6 molecule was carried by newborns as follows: DQ2/DR3-DQ6/DR15, 1.3%; DQ8/DR4-DQ6/DR15, 1.3%; and DQ6/DR15, 9.4%. We conclude from our results that the high incidence of IDDM in Sweden is at least in part due to increased prevalence of high-risk HLA haplotypes compared to protective haplotypes (20% vs. 13%) in the general population.