The Diabetes Control and Complications Trial (DCCT), a multicenter, randomized, controlled clinical trial, demonstrated that intensive diabetes therapy delays the onset and slows the progression of retinopathy, nephropathy, and neuropathy in patients with insulin-dependent diabetes mellitus. This study presents the effect of intensive therapy on atherosclerosis-related events and associated risk factors. Patients (n = 1,441) between the ages of 13 and 39 years with insulin-dependent diabetes mellitus were randomly assigned to conventional or intensive diabetes treatment. The patients were free of cardiovascular disease at baseline. Patients with hypertension, hypercholesterolemia, or obesity were excluded. Average length of follow-up was 6.5 years (range 3.5 to 9). The study used standardized definitions of macrovascular events, verification of such events, and central laboratories for determination of lipids and the grading of electrocardiograms. The number of combined major macrovascular events was almost twice as high in the conventionally treated group (40 events) as in the intensive-treatment group (23 events), although the differences were not statistically significant (p = 0.08). There were no differences in the cumulative incidence of hypertension. Mean total serum cholesterol, calculated low-density lipoprotein cholesterol, and triglycerides were significantly reduced in the intensive-treatment group (p < or = 0.01), as was the development of low-density lipoprotein cholesterol levels > 160 mg/dl. Weight gain was significantly increased in the intensive-treatment group (p < 0.001). There were no differences in cigarette smoking habits, consumption of alcohol, or aspirin use between treatment groups. The reduction in some, but not all, cardiovascular risk factors suggests a potential beneficial effect of intensive therapy on macrovascular disease in insulin-dependent diabetes mellitus.