Insulin dependent diabetes mellitus shows a strong familial predisposition and an unexplained geographical variation in incidence. It is not known whether the risk of IDDM in first degree relatives depends on the risk in the background population. The aim of the study was to assess the prevalence of IDDM in parents and siblings of newly diagnosed children with IDDM in Austria, a known area of low risk for IDDM. The family history data of all diabetic children (< 15 years) diagnosed between 1988-1994 in Austria were analysed. The cumulative incidence of IDDM in siblings of newly diagnosed diabetic children was 0.0026772 cases/year, this means 29.7 times increased risk compared to the background population. Of the diabetic children 5.8% had at least one parent with IDDM and the prevalence of IDDM in fathers (3.9%) was higher (p = 0.015) compared to mothers (1.9%). The risk of IDDM tended to be higher for offsprings of diabetic fathers (OR 3.8, p < 0.003) in families with 2 or more children than in single child families, where the prevalence was 4.2% both in fathers and mothers. In conclusion the prevalence of IDDM in parents of diabetic children in Austria was lower than reported in populations with high IDDM incidence. This may reflect a lesser degree of genetic predisposition of the Austrian population. The prevalence of IDDM in siblings was similar to that in high risk populations. We saw an interaction of gender of the diabetic parent and diabetic offspring and the family size.