Purpose To assess the effect of duration of macular edema (ME) on clinical outcomes after treatment with dexamethasone intravitreal implant 0.7 mg (Ozurdex; Allergan, Inc, Irvine, CA) in patients with ME following branch retinal vein occlusion (BRVO) or central retinal vein occlusion (CRVO). Design Post hoc analysis of pooled data from 2 randomized, controlled trials. Participants Patients with vision loss resulting from ME of 6 weeks' duration or more after BRVO or CRVO (n = 690). Methods The relationship between ME duration at the time of first treatment and treatment outcomes was assessed using logistic regression. Other factors potentially associated with ME duration or patient outcomes were adjusted for in the analyses. Main Outcome Measures The proportion of patients achieving at least 15 letters improvement in best-corrected visual acuity (BCVA) or at least 200-μm or more reduction in central retinal thickness 6 or 12 months after the first treatment with dexamethasone intravitreal implant 0.7 mg. Results In the 6-month analysis, each 1-month increase in ME duration was associated with a significantly lower likelihood of achieving a BCVA improvement of 15 letters or more (odds ratio [OR], 0.88; 95% confidence interval [CI], 0.83–0.94; P<0.001) or a CRT reduction of 200-μm or more (OR, 0.91; 95% CI, 0.86–0.97; P<0.01) 6 months after treatment. In the 12-month analysis, increased ME duration was associated with a significantly lower likelihood of achieving BCVA improvement of 15 letters or more improvement in BCVA (OR, 0.85; 95% CI, 0.76–0.95; P<0.01) 12 months after treatment; duration was not significantly associated with the likelihood of a CRT reduction of 200-μm or more at 12 months. In general, the effect of ME duration on outcomes was stronger and statistically significant in BRVO patients, but weaker and not statistically significant in CRVO patients. Conclusions In eyes with retinal vein occlusion, longer ME duration at the time of first treatment with the dexamethasone intravitreal implant 0.7 mg was associated with a significantly lower likelihood of achieving clinically meaningful improvements in vision or CRT 6 or 12 months after treatment. This suggests that prompt treatment for retinal vein occlusion, particularly BRVO, may be associated with improved clinical outcomes. Financial Disclosure(s) Proprietary or commercial disclosure may be found after the references.