Introduction: Diffuse macular edema is characterized by diffuse leakage from the retinal capillaries and formation of cystoid spaces. In eyes with diffuse macular edema, laser treatment cannot be focused on localized leaking microaneurysms, since there is diffuse leakage from the capillary bed and thickening of the entire macula. In the ETDRS, grid laser treatment was applied to areas of diffuse macular edema. Previous studies have shown, however, that eyes with diffuse macular edema carry a particularly poor prognosis despite laser photocoagulation. It is accepted that in diabetic macular edema there is a breakdown of the blood-retina barrier, and that prostaglandins and VEGF may play a role in this process. A theoretic rationale for the use of corticosteroids in the treatment of diabetic macular edema lies in their ability to inhibit the arachidonic acid pathway, of which prostaglandin is a product, down regulate the production of vascular endothelial growth factor and reduce the breakdown of the blood-retinal barrier.Aim of the study: the aim of our study was to evaluate the efficacy and safety of a single injection of intravitral triamcinolone acetonide in diabetic patients with diffuse macular edema. Methods: this study included thirty eyes of twenty diabetic patients suffering from diffuse macular edema. Fifteen eyes are unresponsive to at least two sessions of macular laser photocoagulation (group I); the other fifteen eyes have not received any laser treatment before the injection of triamcinolone acetonide (group II). All patients were subjected to only one intravitreal injection of triamcinolone acetonide either as a primary method for management of diffuse diabetic macular edema, or following previous failed argon laser photocoagulation. Results: 53.3% of eyes treated by Triamcinolone Acetonide were improved more than one line at the first month, this percentage increased to become 60 % at the third month but declined to 50 % at the last follow up. The mean log MAR of all patients decreased along the follow up period. The decrease of log MAR means improvement of visual acuity. CMT was compared at different intervals and a statistically significant improvement was observed in each group separately throughout the follow up period with a maximal improvement at the third month in group I and at the first month in group II.Conclusion: This study supports the use of intravitreal injection of triamcinolone acetonide in improving vision and reducing macular thickness in all patients with diffuse diabetic macular edema either refractory to laser or as a primary therapy, at least in the short term. Intravitreal injection of TA has many complications. One of the common and major side effects is steroid induced elevation of IOP, starting about 1-2 months after injection. In the vast majority, IOP can be usually normalized by topical anti glaucoma medication. Proper preoperative patient selection in addition to accurate measurement of IOP before and after injection and during the follow up period is therefore mandatory.