Context: Diabetic ketoacidosis and severe hypoglycemia are acute complications of type 1 diabetes that are related, respectively, to insufficient or excessive insulin treatment. However, little is known about additional modifiable risk factors. Objective: To examine the incidence of ketoacidosis and severe hypoglycemia in children with diabetes and to determine the factors that predict these complications. Design, Setting, and Participants: A cohort of 1243 children from infancy to age 19 years with type 1 diabetes who resided in the Denver, Colo, metropolitan area were followed up prospectively for 3994 person-years from January 1, 1996, through December 31, 2000. Main Outcome Measures: Incidence of ketoacidosis leading to hospital admission or emergency department visit and severe hypoglycemia (loss of consciousness, seizure, or hospital admission or emergency department visit). Results: The incidence of ketoacidosis was 8 per 100 person-years and increased with age in girls (4 per 100 person-years in <7; 8 in 7-12; and 12 in ≥13 years; P<.001 for trend). In multivariate analyses, sex-adjusted and stratified by age (<13 vs ≥13 years), the risk of ketoacidosis in younger children increased with higher hemoglobin A1c (HbA1c) (relative risk [RR], 1.68 per 1% increase; 95% confidence interval [Cl], 1.45-1.94) and higher reported insulin dose (RR, 1.40 per 0.2 U/kg per day; 95% Cl, 1.20-1.64). In older children, the risk of ketoacidosis increased with higher HbA1c (RR, 1.43; 95% Cl, 1.30-1.58), higher reported insulin dose (RR, 1.13; 95% Cl, 1.02-1.25), underinsurance (RR, 2.18; 95% Cl, 1.65-2.95), and presence of psychiatric disorders (for boys, RR, 1.59; 95% Cl, 0.96-2.65; for girls, RR, 3.22; 95% Cl, 2.25-4.61). The incidence of severe hypoglycemia was 19 per 100 person-years (P<.001 for trend) and decreased with age in girls (24 per 100 patient-years in <7, 19 in 7-12, and 14 in ≥13 years). In younger children, the risk of severe hypoglycemia increased with diabetes duration (RR, 1.39 per 5 years; 95% Cl, 1.16-1.69) and underinsurance (RR, 1.33; 95% Cl, 1.08-1.65). In older children, the risk of severe hypoglycemia increased with duration (RR, 1.34; 95% Cl, 1.25-1.51), underinsurance (RR, 1.42; 95% Cl, 1.11 -1.81), lower HbA1c (RR, 1.22; 95% Cl, 1.12-1.32), and presence of psychiatric disorders (RR, 1.56; 95% Cl, 1.23-1.98). Eighty percent of episodes occurred among the 20% of children who had recurrent events. Conclusions: Some children with diabetes remain at high risk for ketoacidosis and severe hypoglycemia. Age- and sex-specific incidence patterns suggest that ketoacidosis is a challenge in adolescent girls while severe hypoglycemia continues to affect disproportionally the youngest patients and boys of all ages. The pattern of modifiable risk factors indicates that underinsured children and those with psychiatric disorders or at the extremes of the HbA1c distribution should be targeted for specific interventions.