BACKGROUND: Cardiovascular disease causes severe morbidity and mortality in type 1 diabetes, although the specific risk factors and whether chronic hyperglycemia has a role are unknown. We examined the progression of carotid intima-media thickness, a measure of atherosclerosis, in a population with type 1 diabetes. METHODS: As part of the Epidemiology of Diabetes Interventions and Complications (EDIC) study, the long-term follow-up of the Diabetes Control and Complications Trial (DCCT), 1229 patients with type 1 diabetes underwent B-mode ultrasonography of the internal and common carotid arteries in 1994-1996 and again in 1998-2000. We assessed the intima-media thickness in 611 subjects who had been randomly assigned to receive conventional diabetes treatment during the DCCT and in 618 who had been assigned to receive intensive diabetes treatment. RESULTS: At year 1 of the EDIC study, the carotid intima-media thickness was similar to that in an age- and sex-matched nondiabetic population. After six years, the intima-media thickness was significantly greater in the diabetic patients than in the controls. The mean progression of the intima-media thickness was significantly less in the group that had received intensive therapy during the DCCT than in the group that had received conventional therapy (progression of the intima-media thickness of the common carotid artery, 0.032 vs. 0.046 mm; P=0.01; and progression of the combined intima-media thickness of the common and internal carotid arteries, -0.155 vs. 0.007; P=0.02) after adjustment for other risk factors. Progression of carotid intima-media thickness was associated with age, and the EDIC base-line systolic blood pressure, smoking, the ratio of low-density lipoprotein to high-density lipoprotein cholesterol, and urinary albumin excretion rate and with the mean glycosylated hemoglobin value during the mean duration (6.5 years) of the DCCT. CONCLUSIONS: Intensive therapy during the DCCT resulted in decreased progression of intima-media thickness six years after the end of the trial.