An incidence of 9.0 out of 100,000 children under 19 years of age with Type 1 (insulin-dependent) diabetes was detected in Toronto during a 2 year prospective study. An increased number of cases occurred in the winter months of one of the years but not of the other during the study. The annual incidence was the same in each year. There were slightly more boys than girls but this was not significant. The diabetic children were significantly younger than their non-affected siblings. The incidence of Type 1 diabetes in Toronto is similar to other North American studies.