To assess the outcome of laser photocoagulation in patients with diabetic macular edema. Forty-seven patients (51 eyes) with clinically significant macular edema (CSME) undergoing grid laser photocoagulation were included. Clinical examination and optical coherence tomography (OCT) were performed at baseline and 3 to 4 months after treatment. The central foveal thickness, mean inner macular thickness (average retinal thickness in fovea and inner macular circle), and mean macular thickness were calculated. Based on the greatest OCT thickness at baseline, patients were grouped according to mild (< 300 microm; Group 1), moderate (300 to 399 microm; Group 2), and severe (> or = 400 microm; Group 3) macular edema. Group 2 showed significant reductions in central foveal thickness (23 microm, P = .02), mean inner macular thickness (18 microm, P = .02), and mean macular thickness (9 microm, P = .04) with slight improvement in visual acuity. Groups 1 and 3 did not show any significant change in macular thickness values and there was a statistically insignificant worsening of visual acuity in these groups. Patients with moderate macular thickening of 300 to 400 microm benefit most from laser treatment. OCT may help in choosing the appropriate treatment for CSME based on the degree of macular thickening. Long-term studies are warranted to confirm these findings.