Carriage of a polymorphism in the 3' untranslated region of the IL12B gene encoding IL-12p40 was investigated in subjects with type 1 diabetes mellitus stratified by age at diagnosis (n = 648) and compared with a population-based control cohort (n = 246) residing in Western Australia. DNA samples were genotyped by polymerase chain reaction-restriction fragment length polymorphism or pyrosequencing. The C allele was more common in patients diagnosed after age 16 years than in controls (29% vs 17%, OR = 2.0, 95% CI = 1.4-2.7, p = 0.00003) or than in patients diagnosed when younger age 16 years (29% vs 22176, OR = 1.4, 95% CI = 1.1-1.9, p = 0.01). This reflected increases in homozygous and heterozygous carriage of the C allele. Heterozygosity was associated with a delayed disease in the late-onset diabetics (p = 0.005; Student's t-test). The effects of IL12B 3' untranslated region alleles on type 1 diabetes mellitus may reflect different levels of p40 available to form p40 homodimer, IL-12 (p35p40), and IL-23 (p19p40). (C) American Society for Histocompatibility and Immunogenetics, 2004. Published by Elsevier Inc.