The Diabetes Control and Complications Trial has demonstrated that intensive diabetes treatment delays the onset and slows the progression of diabetic complications in subjects with insulin-dependent diabetes mellitus from 13 to 39 years of age. We examined whether the effects of such treatment also occurred in the subset of young diabetic subjects (13 to 17 years of age at entry) in the Diabetes Control and Complications Trial. One hundred twenty-five adolescent subjects with insulin-dependent diabetes mellitus but with no retinopathy at baseline (primary prevention cohort) and 70 adolescent subjects with mild retinopathy (secondary intervention cohort) were randomly assigned to receive either (1) intensive therapy with an external insulin pump or at least three daily insulin injections, together with frequent daily blood-glucose monitoring, or (2) conventional therapy with one or two daily insulin injections and once-daily monitoring. Subjects were followed for a mean of 7.4 years (4 to 9 years). In the primary prevention cohort, intensive therapy decreased the risk of having retinopathy by 53% (95% confidence interval: 1% to 78%; p = 0.048) in comparison with conventional therapy. In the secondary intervention cohort, intensive therapy decreased the risk of retinopathy progression by 70% (95% confidence interval: 25% to 88%; p = 0.010) and the occurrence of microalbuminuria by 55% (95% confidence interval: 3% to 79%; p = 0.042). Motor and sensory nerve conduction velocities were faster in intensively treated subjects. The major adverse event with intensive therapy was a nearly threefold increase of severe hypoglycemia. We conclude that intensive therapy effectively delays the onset and slows the progression of diabetic retinopathy and nephropathy when initiated in adolescent subjects; the benefits outweigh the increased risk of hypoglycemia that accompanies such treatment.