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 9.1% for conventional treatment vs 7.3% for intensive treatment have been maintained through 7 years of EDIC, even though the difference in mean HbA(1c) levels of the 2 former randomized treatment groups was only 0.4% at 1 year (P<.001) (8.3% in the former conventional treatment group vs 7.9% in the former intensive treatment group), continued to narrow, and became statistically nonsignificant by 5 years (8.1% vs 8.2%, P =.09). The further rate of progression of complications from their levels at the end of the DCCT remains less in the former intensive treatment group. Thus, the benefits of 6.5 years of intensive treatment extend well beyond the period of its most intensive implementation. Intensive treatment should be started as soon as is safely possible after the onset of type 1 diabetes mellitus and maintained thereafter, aiming for a practicable target HbA(1c) level of 7.0% or less.