In a prospective family study in Finland HLA genotyping was carried out for 1610 individuals from 422 consecutively registered families of children aged 14 years or younger with newly diagnosed insulin-dependent diabetes mellitus (IDDM). A haplotype (A2, Cw1, Bw56, w6, DR4) that has not been identified previously was the third most common (5.5%) among 746 haplotypes found in the probands. Only the haplotypes A1, Cw7, B8, w6, DR3 and A2, Cw3, Bw62, w6, DR4, which are well known to occur with high frequency in IDDM, were more frequent (10.7% and 9.7%, respectively). Among the 30 families in which a parent had IDDM, the newly identified haplotype was the most common haplotype transmitted from the diabetic parent to the proband (16.7%). Among 642 "non-diabetic" haplotypes (parental haplotypes not found in probands, or siblings or parents with IDDM), this haplotype was found only twice. This haplotype may partially explain the high incidence of IDDM in Finland.