Aspirin treatment did not alter the course of diabetic retinopathy in patients enrolled in the Early Treatment Diabetic Retinopathy Study (ETDRS). In this randomized clinical trial supported by the National Eye Institute, 3711 patients with mild-to-severe nonproliferative or early proliferative diabetic retinopathy were assigned randomly to either aspirin (650 mg per day) or placebo. Aspirin did not prevent the development of high-risk proliferative retinopathy and did not reduce the risk of visual loss, nor did it increase the risk of vitreous hemorrhage. This was true both for eyes assigned randomly to deferral of photocoagulation and for eyes assigned randomly to early argon laser photocoagulation. The ETDRS results indicate that for patients with mild-to-severe non-proliferative or early proliferative diabetic retinopathy, it is likely that aspirin has no clinically important beneficial effects on the progression of retinopathy. The data also show that aspirin 650 mg per day had no clinically important harmful effects for diabetic patients with retinopathy. These findings suggest there are no ocular contraindications to aspirin when required for cardiovascular disease or other medical indications.