This study was undertaken to evaluate whether differences in Type 1 diabetes incidence in two separate geographical locations are associated with any differences in biochemical and immunological features at the onset of the disease. We studied all children under 16 years of age who presented at the time of diagnosis with Type 1 diabetes to the children's hospitals in Oulu (n = 43) and Toronto (n = 87) during 1984-1985. At onset children from Northern Finland had lower blood glucose and HbAlc levels, and higher C-peptide concentrations than those from Southern Ontario (p's less than 0.01). The group from Northern Finland also had a higher incidence of multiplex families (18.6 vs 4.6%, p less than 0.01). Amongst the Finnish group, those from multiplex families had a higher C-peptide concentration and lower frequency of ketoacidosis than those from simplex families (p's less than 0.05). Thus differences do exist at the onset of diabetes in these groups from geographical locations with greatly different incidences of Type 1 diabetes.