The Early Treatment Diabetic Retinopathy Study (ETDRS) has recently shown that argon laser photocoagulation treatment is beneficial in reducing the risk of visual loss from clinically significant diabetic macular edema. The ETDRS treatment consisted of a combination of focal treatment to individual-leaking microaneurysms and grid treatment to areas of diffuse leakage and capillary nonperfusion. These techniques are described in detail, and the concepts of "clinically significant macular edema" and "treatable lesions" are defined. Guidelines for the application of ETDRS findings to clinical practice are discussed.