The Early Treatment Diabetic Retinopathy Study (ETDRS) enrolled 3711 patients with mild-to-severe nonproliferative or early proliferative diabetic retinopathy in both eyes. One eye of each patient was assigned randomly to early photocoagulation and the other to deferral of photocoagulation. Follow-up examinations were scheduled at least every 4 months and photocoagulation was initiated in eyes assigned to deferral as soon as high-risk proliferative retinopathy was detected. Eyes selected for early photocoagulation received one of four different combinations of scatter (panretinal) and focal treatment. This early treatment, compared with deferral of photocoagulation, was associated with a small reduction in the incidence of severe visual loss (visual acuity less than 5/200 at two consecutive visits), but 5-year rates were low in both the early treatment and deferral groups (2.6% and 3.7%, respectively). Adverse effects of scatter photocoagulation on visual acuity and visual field also were observed. These adverse effects were most evident in the months immediately following treatment and were less in eyes assigned to less extensive scatter photocoagulation. Provided careful follow-up can be maintained, scatter photocoagulation is not recommended for eyes with mild or moderate nonproliferative diabetic retinopathy. When retinopathy is more severe, scatter photocoagulation should be considered and usually should not be delayed if the eye has reached the high-risk proliferative stage. The ETDRS results demonstrate that, for eyes with macular edema, focal photocoagulation is effective in reducing the risk of moderate visual loss but that scatter photocoagulation is not. Focal treatment also increases the chance of visual improvement, decreases the frequency of persistent macular edema, and causes only minor visual field losses. Focal treatment should be considered for eyes with macular edema that involves or threatens the center of the macula.