Data from the Early Treatment Diabetic Retinopathy Study (ETDRS) show that focal photocoagulation of "clinically significant" diabetic macular edema substantially reduces the risk of visual loss. Focal treatment also increases the chance of visual improvement, decreases the frequency of persistent macular edema, and causes only minor visual field losses. In this randomized clinical trial, which was supported by the National Eye Institute, 754 eyes that had macular edema and mild to moderate diabetic retinopathy were randomly assigned to focal argon laser photocoagulation, while 1,490 such eyes were randomly assigned to deferral of photocoagulation. The beneficial effects of treatment demonstrated in this trial suggest that all eyes with clinically significant diabetic macular edema should be considered for focal photocoagulation. Clinically significant macular edema is defined as retinal thickening that involves or threatens the center of the macula (even if visual acuity is not yet reduced) and is assessed by stereo contact lens biomicroscopy or stereo photography. Follow-up of all ETDRS patients continues without other modifications in the study protocol.