OBJECTIVE To compare argon green (514 nm) versus diode laser (810 nm) modified grid laser photocoagulation treatment in diffuse diabetic macular edema (DDME). METHODS Randomized, prospective clinical trial. METHODS Patients with DDME and diabetic retinopathy of fewer than two high-risk characteristics in severity, no previous laser photocoagulation for diabetic macular edema, and no other ocular condition that could interfere with assessment of treatment results. METHODS One hundred seventy-one eyes of 91 patients were randomized to either argon green (514 nm) or diode laser (810 nm) modified grid laser photocoagulation for DDME. Follow-up was conducted for a minimum of 12 months (16.55 +/- 3.52 months). Retreatment was performed for residual edema involving the foveal avascular zone. RESULTS Visual improvement, visual loss, reduction-elimination of macular edema, and the number of supplemental treatments. RESULTS A comparison of visual improvement, visual loss, reduction-elimination of macular edema, and the number of supplemental treatments showed no statistical difference between the groups (P > 0.05 for all groups). Reduction-elimination of DDME was better in the group without cystoid macular edema than the group with cystoid macular edema, but visual outcome appeared to be similar in both groups. History of hypertension or poor initial visual acuity (< or = 20/80) at entry into the study had no significant effect on the outcome. However, the patients without systemic vascular disease have improved more than those with systemic vascular disease regardless of the type of the laser used. CONCLUSIONS Diode laser (810 nm) modified grid laser photocoagulation for DDME is equivalent to argon green (514 nm) and patients without systemic vascular disease are more likely to improve after laser treatment with either wavelength.