PURPOSE: To assess the effect of a single intraoperative sub-Tenon injection of triamcinolone acetonide on the progression of diabetic retinopathy (DR), visual outcomes, and cystoid macular edema (CME) after cataract surgery. SETTING: Department of Ophthalmology and Visual Science, College of Medicine, The Catholic University of Korea, Seoul, Korea. METHODS: This prospective controlled study comprised 46 eyes of 23 diabetic patients. The control group comprised eyes not treated with a triamcinolone acetonide injection, and the triamcinolone group comprised contralateral eyes treated with a single posterior sub-Tenon injection of triamcinolone acetonide at the end of cataract surgery. The visual acuity, central macular thickness (CMT) by optical coherence tomography, and progression of DR were compared between the 2 groups. RESULTS: One month postoperatively, the mean change in lines of best corrected visual acuity was significantly greater in the triamcinolone group (P = .045) and the mean change in CMT was significantly greater in the control group (P = .015). The incidence of CME was higher in the control group (P = 0.04). Scores for progression of DR at 6 months were not statistically significantly different between the 2 groups (P = 0.08). CONCLUSIONS: A posterior sub-Tenon injection of triamcinolone acetonide lowered the incidence of CME after cataract surgery in diabetic patients, improved visual recovery, and reduced the amount of CMT increase in the short term (<= 1 month postoperatively). However, triamcinolone acetonide did not affect DR progression over the 6-month follow-up.