BACKGROUND Glycosylated haemoglobin (HbA1c) correlates with the amount of hyperglycemia in diabetic patients. High HbA1c levels often predict clinically significant macular edema (CSME), which then needs to be treated with grid laser photocoagulation. The question asked in this study was whether there is a correlation between the effect grid laser photocoagulation in diffuse diabetic macular edema and HbA1c, using an optical coherence tomography (OCT) for the evaluation of the retinal thickness. METHODS A prospective, non-comparative case series was performed to find a correlation between the effect of grid laser photocoagulation in diffuse diabetic macular edema and HbA1c. Thirty eyes with CSME of diabetic patients were included in the study. Complete ophthalmic examinations and OCT were performed at baseline, 1 month, 3, and 6 months after grid laser photocoagulation therapy. HbA1c was measured at the end of study. RESULTS Significance level was set at P<0.05. A significant difference in the foveal (P=0.02) and superior (P=0.021) retinal thickness 6 months after laser therapy, no correlation between HbA1c and retinal thickness after photocoagulation, and an insignificant decrease in visual acuity (P=0.9) were found. The correlation between foveal retinal thickness and visual acuity was P=0.24 6 months after treatment. CONCLUSIONS There was no significant correlation between HbA1c and the effect of grid laser photocoagulation therapy in diffuse diabetic macular edema. The retinal thickness decreased significantly in the foveal and superior area 6 months after therapy. No correlation between the foveal retinal thickness and the visual acuity was found. The visual acuity did not increase after treatment. There are many factors influencing the retinal thickness, such as the blood pressure and the attached posterior hyaloid.