Purpose: To investigate the efficacy of intravitreal triamcinolone (IVT) by evaluation of pattern electroretinogram (PERG) in diabetic patients with clinically significant macular edema (CSME). Methods: Forty eyes of 40 diabetic patients were treated with 8 mg of IVT injection as primary therapy for CSME. The main outcome measures included best-corrected visual acuity, fundus fluorescein angiography, P₅₀ amplitudes of pattern electroretinogram (PERG) and intraocular pressures before and after injection. Results: The mean follow-up time was 6.1 months. Mean visual acuity improved significantly from a mean LogMAR value of 1.14 ± 0.16 at baseline to a maximum of 0.73 ± 0.30. The mean baseline P₅₀ amplitude of PERG before intravitreal injection was 1.5 ± 0.9 V. After the treatment, it was 2.l ± 1.1 V at 1-month, 2.4 ± 1.0 V at 3-month and 2.1 ± 0.9 V at last visit and the differences were significant when compared with baseline values (for each, p < 0.001). Temporary increases in intraocular pressures were controlled with topical antiglaucomatous medications, if required. Conclusion: IVT injection provides rapid improvement in visual acuity of diabetic patients with CSME that has been supported by P₅₀ amplitudes of PERG. P₅₀ amplitudes of PERG may be used as novel predictive value in the evaluation of the effectiveness of IVT injection.