To compare eyes that received treatment with untreated fellow eyes to assess the effectiveness of injection of 20 mg of triamcinolone acetonide (TA) into the posterior sub-Tenon capsule for treatment of diabetic macular edema (DME). Nine adult patients (mean age, 60.4 years) with DME in both eyes were followed up for 3 months after unilateral treatment. We compared central macular thickness before and after treatment with those of the untreated fellow eye. In 9 eyes injected with 20 mg of TA, the mean central macular thickness +/- SD was 624.8 +/- 173.7 microm before treatment. In the fellow eyes, it was 452.8 +/- 235.2 microm. There was no significant difference between the two groups (P=0.10, unpaired t-test). One month after injection, it was 434.0 +/- 120.7 microm in injected eyes (P=0.017, paired t-test) and 462.2 +/- 232.7 microm in fellow eyes (P=0.70, paired t-test), and after 3 months, the respective values were 423.1 +/- 186.3 microm (P=0.052, paired t-test) and 478.3 +/- 269.1 microm (P=0.65, paired t-test). Injection of 20 mg of TA into the posterior sub-Tenon capsule effectively reduces macular thickening due to DME, at least in the short term.