Analyses of visual acuity and visual field results in the Diabetic Retinopathy Study provide evidence that photocoagulation treatment as carried out according to the study protocol (extensive "scatter" photocoagulation and focal treatment of new vessels) is of benefit in preventing severe visual loss, over a two-year follow-up period, in eyes with proliferative retinopathy. Location of new vessels relative to the disk, severity on new vessels, and the presence of hemorrhage (vitreous or preretinal) all proved to be important prognostic factors. On the basis of these findings, these steps have been taken: All patients in the study have been informed of results to date and given an explanation of their implications. Photocoagulation treatment will be considered for the initially untreated eyes which now or in the future fulfill any one of the following criteria: (a) moderate or severe new vessels on or within 1-disk diameter of the optic disk; (b) mild new vessels on or within 1-disk diameter of the optic disk if fresh hemorrhage is present; and (c) moderate or severe new vessels elsewhere, if fresh hemorrhage is present. Follow-up of all patients will continue to allow long-term comparison between the argon- and xenon-treatment techniques employed. Further analyses of accumulating data will be performed to evaluate more completely the efficacy of photocoagulation therapy.