OBJECTIVE During theDiabetes Control and Complications Trial (DCCT), intensive diabetes therapy achieving a mean HbA1c of ?7% was associated with a threefold increase in the rate of severe hypoglycemia (defined as requiring assistance) compared with conventional diabetes therapywith ameanHbA1c of9%(61.2 vs. 18.7 per 100 patient-years). After?30 years of follow-up,we investigated the rates of severe hypoglycemia in the DCCT/Epidemiology of Diabetes Inverventions and Complications (EDIC) cohort. RESEARCH DESIGN AND METHODS Rates of severe hypoglycemia were reported quarterly during DCCT and annually during EDIC (i.e., patient recall of episodes in the preceding 3 months). Risk factors influencing the rate of severe hypoglycemia over time were investigated. RESULTS One-half of the DCCT/EDIC cohort reported episodes of severe hypoglycemia. During EDIC, rates of severe hypoglycemia fell in the former DCCT intensive treatment group but rose in the former conventional treatment group, resulting in similar rates (36.6 vs. 40.8 episodes per 100 patient-years, respectively) with a relative risk of 1.12 (95% CI 0.91-1.37). A preceding episode of severe hypoglycemia was the most powerful predictor of subsequent episodes. Entry into the DCCT study as an adolescent was associated with an increased risk of severe hypoglycemia,whereas insulin pump use was associated with a lower risk. Severe hypoglycemia rates increased with lower HbA1c similarly among participants in both treatment groups. CONCLUSIONS Rates of severe hypoglycemia have equilibrated over time between the two DCCT/ EDIC treatment groups in association with advancing duration of diabetes and similar HbA1c levels. Severe hypoglycemia persists and remains a challenge for patientswith type 1 diabetes across their life span.