Epidemiological data are presented to support the hypothesis that the genetic contribution to disease pathogenesis in childhood onset diabetes is greatest in those presenting at a very early age. Analysis of family data from two national surveys of childhood onset Type 1 (insulin-dependent) diabetes (1988 in under 15s: 1992 in under 5s) reveals that children developing diabetes between the ages 1 and 2 years are significantly more likely to have a parental history of Type 1 diabetes than older children. It is proposed that compared with other children, those with very early onset diabetes have either a greater genetic and smaller environmental contribution to the initiation of the autoimmune process leading to Type 1 diabetes: or inherited HLA alleles associated with a more fulminant autoimmune mediated Beta-cell destruction once the process is initiated.