The aim of the study was to determine the clinical and biochemical characteristics of type 1 diabetes mellitus (DM) at presentation in children younger than 15 years in Croatia during a 9-year period, with special attention to diabetic ketoacidosis (DKA) incidence. The registered data set comprised blood glucose, pH, serum bicarbonate levels, and clinical symptoms at disease manifestation. During the study period, 692 children were diagnosed with type 1 DM. Polydipsia (96.7%), polyuria (96.05%), and weight loss (82.7%) were the most frequent symptoms anticipating disease detection. Enuresis was recorded in 11.55%. A total of 36.41% patients had DKA (pH < 7.3) at disease onset. During the 9-year period, the percentage of children presenting with DKA at time of diagnosis decreased from 41.67% to 33.33% (z = 1.68, p = 0.046). A positive family history of DM, the only factor with an impact on the DKA incidence rate in our population, lowers the probability of the development of ketoacidosis. This study confirms the importance of the detection of the classic symptoms of polyuria, polydipsia, and weight loss in patients with new-onset type 1 DM. The percentage of patients with DKA at diabetes onset decreased during the observed period but is still high and includes one-third of all patients. This is why in every acutely ill child, especially at a younger age, one should evaluate the possibility of type 1 DM to avoid the development of ketoacidosis.