Haplotypes including HLA, Bf and C4 loci were analyzed in a material comprising 55 families with diabetic children. One hundred and ten haplotypes found in IDDM patients were compared with 101 haplotypes present only in healthy family members. Two complotypes, BfSC4A3B3 and SC4A0B1, were significantly more common (P less than 0.05) in the diabetic haplotypes, and these were in most cases found in haplotypic combinations with HLA-B15,Dw4,DR4 and HLA-B8,Dw3,DR3 genes, respectively. The B8/DR3 haplotype was better conserved, as 72% included the BfSC4A0B1 complotype as compared with only 35% of the B15/DR4 haplotypes with "high risk" C4A3B3 complement alleles (p less than 0.05). DR3 was found in 26% of the diabetic haplotypes and DR4 in 43%. DR4 associated with the Dw4 in 69% of cases and with Dw14 in 26% of the diabetic haplotypes. Our results confirm that the two phenotypes found earlier to be associated with IDDM in Northern Finland, e.g. "B15,BfS,C4A3B3,Dw4,DR4" and "B8,Bfs,C4A0B1,Dw3,DR3" are inherited as haplotypes.