French health insurance data showed that the incidence of type 1 diabetes mellitus (T1DM) in children increased over the years to 2015. The objective of our study was to assess the evolution of the number of incident cases of paediatric and adult type 1 diabetes in our institution, and to describe their clinical presentation and its evolution. All patients with T1DM managed at diagnosis at Reims University Hospital between 1997 and 2019 were included. The clinical and biological data were extracted from the Champagne-Ardenne Diabetes Network database. Included were 847 patients with a median age of 10.3 years. Diagnosis was established in 71% of cases before 15 years, 7.4% after 35 years. The number of newly diagnosed cases was 3.6-times higher in 2019 compared to 1997. Ketoacidosis, the frequency of which decreased with age (P < 0.0001), revealed diabetes in a total of 32% of cases and in 46% of children under 5 years. It was more severe in children than in adults (P = 0.03), and its frequency increased over the study period. Hypotrophy was found in 23% of children under 15 years of age, and was more pronounced before 5 years of age, with no improvement over time. We saw an increase in the frequency of obesity or overweight among adults. Our study showed an increase in incident cases of diabetes in our hospital that continued over time for both children and adults. Clinical features at diagnosis deteriorated during this period for those under 15 years of age with an increase in ketoacidosis frequency.