The purpose of this report is to summarize and integrate the findings of the Diabetes Control and Complications Trial (DCCT), a randomized controlled clinical trial, and the succeeding observational follow-up of the DCCT cohort in the Epidemiology of Diabetes Interventions and Complications (EDIC) study, regarding the effects of intensive treatment on the microvascular complications of type 1 diabetes mellitus. The DCCT proved that intensive treatment reduced the risks of retinopathy, nephropathy, and neuropathy by 35% to 90% compared with conventional treatment. The absolute risks of retinopathy and nephropathy were proportional to the mean glycosylated hemoglobin (HbA 1c ) level over the follow-up period preceding each event Intensive treatment was most effective when begun early, before complications were detectable. These risk reductions, achieved at a median HbA 1c level difference of 9