Between 1984 and 1988, 225 eyes of 132 patients were entered in a prospective, randomized clinical trial to determine if any significant differences exist between treatment with argon green (514 nm) and krypton red (647 nm) modified grid laser photocoagulation for patients with diffuse diabetic maculopathy with or without cystoid macular edema. At the 12- and 24-month follow-up visits, no statistically significant difference was found between the two groups with respect to all of the following: reduction or elimination of macular edema, improvement in visual acuity, worsening of visual acuity, number of treatments per eye, and effect on the visual field. At the 12-month follow-up visit, none of the following factors statistically affected the visual outcome in either of the two groups: a history of systemic hypertension, systemic vascular disease, cystoid macular edema, or initial poor visual acuity.