The Early Treatment Diabetic Retinopathy Study (ETDRS), a multicenter collaborative clinical trial supported by the National Eye Institute, was designed to assess whether argon laser photocoagulation or aspirin treatment can reduce the risk of visual loss or slow the progression of diabetic retinopathy in patients with mild-to-severe nonproliferative or early proliferative diabetic retinopathy. The 3711 patients enrolled in the ETDRS were assigned randomly to either aspirin (650 mg per day) or placebo. One eye of each patient was assigned randomly to early argon laser photocoagulation and the other to deferral of photocoagulation. Both eyes were to be examined at least every 4 months and photocoagulation was to be initiated in eyes assigned to deferral as soon as high-risk proliferative retinopathy was detected. Examination of a large number of baseline ocular and patient characteristics indicated that there were no important differences between randomized treatment groups at baseline.