Aim: The study aimed to assess the effects of diabetes-related risk factors, especially severe hypoglycaemia, on the academic skills of children with early-onset type 1 diabetes mellitus (T1DM). Method: The study comprised 63 children with T1DM (31 females, 32 males; mean age 9y 11mo, SD 4mo) and 92 comparison children without diabetes (40 females, 52 males; mean age 9y 9mo, SD 3mo). Children were included if T1DM had been diagnosed before the age of 5 years and if they were aged between 9 and 10 years at the time of study. Children were not included if their native language was not Finnish and if they had a diagnosed neurological disorder that affected their cognitive development. Among the T1DM group, 37 had and 26 had not experienced severe hypoglycaemia and 26 had avoided severe hypoglycaemia. Severe hypoglycaemia, diabetic ketoacidosis (DKA), and glycaemic control were used as T1DM-related factors. Task performance in reading, spelling, and mathematics was compared among the three groups, and the effects of the T1DM-related factors were analysed with general linear models. Results: The groups with (p less than 0.001) and without (p = 0.001) severe hypoglycaemia demonstrated a poorer performance than the comparison group in spelling, and the group without severe hypoglycaemia showed a poorer performance than the comparison group in mathematics (p = 0.003). Severe hypoglycaemia, DKA, and recent glycaemic control were not associated with poorer skills, but poorer first-year glycaemic control was associated with poorer spelling (p = 0.013). Interpretation: An early onset of T1DM can increase the risk of learning problems, independently of the history of severe hypoglycaemia or DKA. Poorer glycaemic control after the first year of T1DM is associated with a poorer acquisition of academic skills indicating the effect of the timing of metabolic aberrations on cognitive development.