We aimed at investigating the efficacy of intravitreal bevacizumab injection by evaluation of pattern electroretinogram (PERG) in diabetic patients with clinically significant macular edema (CSME). Thirty-five eyes of 35 patients with diabetes were treated with 2.5 mg of intravitreal bevacizumab injection as the primary therapy for CSME. The main outcome measures included best-corrected visual acuity, fundus fluorescein angiography, and P(50) amplitudes of pattern electroretinogram (PERG) before and after intravitreal injection. Mean visual acuity improved significantly from a mean LogMAR value of 1.1 +/- A 0.2 at baseline to a maximum of 0.7 +/- A 0.3 after a mean follow-up time of 6.4 months. The mean baseline P(50) and N(95) amplitudes of PERG before intravitreal injection were 1.4 +/- A 0.7 and 2.4 +/- A 0.8 mu V, respectively. After the treatment, the mean P(50) amplitude of PERG was 2.4 +/- A 0.9 mu V at 1-month, 2.3 +/- A 0.8 mu V at 3-month, and 2.2 +/- A 0.8 mu V at the last visit, and the mean N(95) amplitude of PERG was 3.4 +/- A 1.05 mu V at 1-month, 3.3 +/- A 0.9 mu V at 3-month, and 3.2 +/- A 0.9 mu V at the last visit, and the mean P(50) and N(95) amplitudes were significantly higher when compared with baseline values (for each, P < .001). Mild anterior chamber inflammation in four eyes was controlled with topical corticosteroids. Eyes with diabetic macular edema treated with intravitreal bevacizumab showed improvement in visual acuity that was accompanied by improvement in PERG amplitudes. Further randomized trials are needed to investigate the role of PERG measurements in the evaluation of the efficacy of intravitreal bevacizumab injection.