Insulin autoantibody (IAA) and islet cell antibody (ICA) titres were measured in 108 newly diagnosed type I diabetics (49 male, 59 female, mean age 20 [1-38] years) and 103 non-diabetic controls (41 male, 62 female, mean age 23 [16-46] years). IAA titres in the controls were normally distributed, with a mean of 5 +/- 11 nU/ml. The upper limit of normal was established as 49 nU/ml (mean + 4 standard deviations). Raised IAA and ICA titres were present in 45% and 44% of type I diabetics, respectively, with 59% positive for either IAA or ICA or both. IAA were markedly age-dependent, being positive in 70% (26 out of 37) of diabetics under the age of 15 years, and in 32% (23 out of 71) at the age of 15 years or more (P = 0.0004). There was a less marked difference for ICA titres (positive in 62% of patients less than 15 years, and in 35% of those of 15 years of older; P less than 0.01). IAA were significantly more common in HLA DR4 positive patients than in HLA DR4 negative patients (56% vs 11%; Pc less than 0.00015). With regard to age a significant association between IAA and HLA DR phenotype was present only in homozygous (Pc less than 0.03) and heterozygous (P less than 0.0003) patients aged 15 years or older. By contrast, ICA was not significantly correlated with HLA phenotype. These data suggest a genetic predisposition for the development of IAA.