BACKGROUND: Children of Sardinian heritage are at high risk of type 1 diabetes, whereas no data are available in young adults. Age at onset of type 1 diabetes could be associated with different relative weight of genetic susceptibility and environmental determinants in the pathogenesis of the disease. We test this hypothesis in subjects with Sardinian heritage 0-29 years of age living in the city of Turin, a highly industrialized area in Northern Italy. METHODS: In all, 202 cases with onset of type 1 diabetes aged 0-29 years during 1984-1991 and 1010 controls randomly selected from residents of the city of Turin, frequency-matched by sex and year of birth to cases, were included in this study. Name and place of birth of parents were ascertained by postal inquiry and linkage with city population and census files. Social class was based on the highest educational level of parents abstracted from 1991 and 1981 census files. RESULTS: Differential effects on risk of type 1 diabetes of Sardinian heritage and social class in the age groups 0-14 and 15-29 years were found. In children with one and both Sardinian parents the odds ratios (OR) were 2.09 (95% CI : 0.85-5.15) and 3.20 (95% CI : 0.75-13.64); in young adults 0.81 (95% CI : 0.18-3.64) and 1.95 (95% CI : 0.51-7.40), respectively. In subjects with low social class the OR were 1.16 (95% CI : 0.68-1.97) in children and 0.66 (95% CI : 0.41-1.05) in young adults. CONCLUSIONS: This study shows higher risk of type 1 diabetes in subjects of Sardinian heritage; higher risk in children than in young adults and a protective effect of low social class in young adults. These findings are consistent with the hypothesis of heterogeneity of type 1 diabetes by age at onset, with prevailing genetic effect in childhood and environmental determinants in adulthood.