To determine the impact of changes to treatment on the incidence of severe hypoglycemia and its risk factors in a large population-based cohort of children with type 1 diabetes. The cohort consisted of 1,335 children (age at entry 9.5 ± 4.3 years [mean ± SD], range 0–18), yielding 6,928 patient-years of data. The mean follow-up period was 4.7 ± 3.1 years (range 0–10.7). Prospective assessment of severe hypoglycemia (an event leading to loss of consciousness or seizure) and associated clinical factors and outcomes was made between 1992 and 2002. Patients were reviewed every 3 months. Data were analyzed using the negative binomial regression model. A total of 944 severe events were recorded. The incidence of severe hypoglycemia increased significantly by 29% per year for the first 5 years but appeared to plateau over the last 5 years. The overall average HbA1c significantly decreased (by 0.2% per year) over the whole follow-up period. An increased risk of severe hypoglycemia was associated with lower HbA1c, younger age, higher insulin dose, male sex, and lower parental socioeconomic status. Of insulin therapies, only pump treatment was associated with reduced rates of severe hypoglycemia. Severe hypoglycemia remains a major problem for children and adolescents with type 1 diabetes. Recent approaches to therapy may be allowing a degree of improved control without the expected increased risk of severe hypoglycemia but further monitoring will be important.