OBJECTIVE To investigate the long-term results of grid pattern photocoagulation for diffuse diabetic macular edema. METHODS In 45 cases, 56 eyes foveal thickness and visual outcome were observed 12 months or more after grid pattern photocoagulation. RESULTS Compared with preoperative values, average foveal thickness decreased significantly during the initial 3 postoperative months and this reduction was maintained through 30 postoperative months. Foveal thickness was reduced by more than 20% in 66% of eyes. The final logarithm of the minimum angle of resolution (logMAR) visual acuity improved 0.2 or more in 41% of eyes. There was a significant correlation between preoperative and postoperative visual acuity. A final visual acuity of 0.5 or better was obtained in 80% of eyes with a preoperative visual acuity of 0.4 or better. There were no eyes with visual loss due to atrophic creep in this study. CONCLUSIONS Grid pattern photocoagulation for diffuse diabetic macular edema showed long-term effectiveness in reducing foveal thickness postoperatively. It will be possible to achieve a final visual acuity of 0.5 or better in a high percentage of cases, if grid pattern photocoagulation is performed for those with relatively good visual acuity. The foveal thickness measurement was useful for quantitative evaluation of photocoagulation for diabetic macular edema.