Purpose: To evaluate prospectively the efficacy of a single sub-Tenon’ s capsule injection of triamcinolone acetonide (TA) against panretinal photocoagulation (PRP)-induced macular thickening and visual disturbance in patients with severe diabetic retinopathy and good vision. Design: Prospective, comparative, interventional case series. Participants: Twenty eyes of 10 patients with severe nonproliferative diabetic retinopathy or non-high-risk proliferative diabetic retinopathy whose visual acuity was 20/40 or better (< 0.3 in logarithm of the minimum angle of resolution LogMAR acuity) before the PRP, whose retinopathy was bilateral and symmetrical. The averaged parafoveal retinal thickness was more than 300 μ m, leading to a worse visual prognosis after PRP. Intervention: Sub-Tenon’ s capsule injection of 20 mg TA. Main Outcome Measures: Best-corrected visual acuity (BCVA) with logMAR chart and averaged foveal thickness (FT) using the retinal mapping program of optical coherence tomography. Methods: In all patients, PRP was performed every other week for 4 sessions on both eyes, and 1 week before PRP; 1 eye received the TA injection, and the other eye served as a control. The clinical course of BCVA and FT was monitored for up to 24 weeks after beginning PRP. Results: Before TA injection, BCVA and FT were 0.055± 0.072 and 235.5± 37.5 μ m in the TA-injected eye and 0.065± 0.071 and 233.7± 39.8 μ m in the control eye, respectively; there was no significant difference between eyes. After the TA injection, FT in the TA-injected eyes was significantly decreased. During and after the PRP, FT in the control eye increased dramatically and reached 312.0± 68.2 μ m at 24 weeks, which was significantly different from that in the TA-injected eyes (235.3± 38.6 μ m at 24weeks). Best-corrected visual acuity in the control eye decreased with time to 0.24± 0.13; in contrast, and BCVA in the TA-injected eye was good (to 0.085± 0.11) . Conclusions: As a pretreatment for PRP, a single sub-Tenon’ s capsule injection of TA has beneficial effects for preventing PRP-induced foveal thickening and visual dysfunction in patients with severe diabetic retinopathy and good vision. Purpose: To evaluate prospectively the efficacy of a single sub-Tenon ’capsule injection of triamcinolone acetonide (TA) against panretinal photocoagulation (PRP) -induced macular thickening and visual disturbance in patients with severe diabetic retinopathy and good vision. Design: Prospective, Participants: Twenty-eyes of 10 patients with severe nonproliferative diabetic retinopathy or non-high-risk proliferative diabetic retinopathy whose visual acuity was 20/40 or better (<0.3 in logarithm of the minimum angle of resolution Log MAR acuity) before the PRP, whose retinopathy was bilateral and symmetrical. The averaged parafoveal retinal thickness was more than 300 μ m, leading to a worse visual prognosis after PRP. Intervention: Sub-Tenon ’s capsule injection of 20 mg TA. Main Outcome Measures: Best-corrected visual acuity (BCVA) with logMAR chart and averaged foveal thickness (FT) using the retinal mapping program of optical coherence t Methods: In all patients, PRP was performed every other week for 4 sessions on both eyes, and 1 week before PRP; 1 eye received the TA injection, and the other eye served as a control. The clinical course of BCVA and FT was monitored for up to 24 weeks after beginning PRP. Results: Before TA injection, BCVA and FT were 0.055 ± 0.072 and 235.5 ± 37.5 μ in the TA-injected eye and 0.065 ± 0.071 and 233.7 ± 39.8 μ in the control eye During and after the PRP, FT in the control eye increased dramatically and reached 312.0 ± 68.2 μ m at 24 weeks, respectively; there was no significant difference between eyes. , which was significantly different from that in TA-injected eyes (235.3 ± 38.6 μm at 24 weeks). Best-corrected visual acuity in the control eye decreased with time to 0.24 ± 0.13; in contrast, and BCVA in the TA-injected eye was good (to 0.085 ± 0.11). Conclusions: As a pretreatment fo r PRP, a single sub-Tenon ’s capsule injection of TA has beneficial effects for preventing PRP-induced foveal thickening and visual dysfunction in patients with severe diabetic retinopathy and good vision.