Objective: The goal of this paper (thesis) is to specify the concentrations of HbA1c in children both with diabetic ketoacidosis and without diabetic ketoacidosis following the first and the second year after the diagnosis had been set. Design: A retrospective cohort study was conducted. Place of research: The research was conducted at the Clinical hospital department for endocrinology of the Children's clinic of CHC Split. Participants: The participants of the study were the children hospitalized and under treatment at the Clinical department for endocrinology of the Children's clinic of CHC Split, from January 1992 to December 2009. The total number of patients included in the research was 242. All the necessary data were collected by inquiry into the medical documentation, charts and medical history. The main measure of the outcome: Concentration of HbA1c Results: A significant decrease of concentration of HbA1c has been established during the treatment of all children together. Averaged concentration of HbA1c per child during the first year of the treatment is 0,61% higher in the group of children with diabetic ketoacidosis than in the group without diabetic ketoacidosis; also, a faster decrease of concentration of HbA1c during the first year in the group of participants without diabetic ketoacidosis has been established. Conclusion: The research has shown that diabetic ketoacidosis adversely influences the metabolic control evidenced through the increase of concentration of HbA1c with patients diagnosed, upon reception, with diabetic ketoacidosis at Universoty department of endocrinology of the Children's clinic of CHC Split.