In more than 80% of cases, IDDM occurs in children with no family history for the disease. In these cases, the prevalence of IDDM is less than 1%. In the remaining 20%, the prevalence is 1% - 15% depending on their genetic background.

Prevalence of IDDM by age 30 years in siblings of IDDM patients is about 3%-6%, whereas IDDM prevalence in the general population is less than 0.2% at age 20 years. Prevalence in parents of IDDM patients is also about 3%-6%. For children with IDDM who have an IDDM parent, the father is more likely to have the disease than the mother. The prevalence of paternal compared with maternal IDDM was 6.2% versus 2.3% in Sweden and 5.7% versus 2.6% in Finland. Prospective studies that ascertained IDDM in the offspring of parents with IDDM have also revealed a higher risk of IDDM in children of affected fathers than mothers, ~6% and 2%, respectively.