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(1) To propose the use of episcleral venous outflow (EVO) as an outcome marker of iStent surgery, and an EVO grading scale. (2) To determine the association of EVO with: (a) postoperative intraocular pressure (IOP) and medication burden; (b) iStent patency status.Retrospective cohort study including 151 glaucomatous eyes having undergone iStent-phacoemulsification surgery. Demographic and preoperative data (IOP, number of antiglaucoma medications, glaucoma type and stage, maximal IOP) were collected. Postoperatively, were recorded: IOP, number of antiglaucoma medications, occurrence of stent malpositioning or obstruction, and EVO scores based on the proposed scale (0: no laminar flow; 1+: faint laminar flow; 2+: marked laminar flow). A Kruskal-Wallis test determined the association between EVO, postoperative IOP, and medication burden. A multivariable-adjusted ordinal logistic regression was used for the association with iStent patency status.Patients with marked laminar flow (2+) were more likely to have a lower IOP (P=0.022) and fewer medications (P=0.009) at 1-year postoperatively than those with no laminar flow (0). No difference was found in postoperative IOP and number of medications when comparing patients having faint laminar flow (1+) with patients from the other 2 EVO categories (0 and 2+). iStent patency was associated with greater EVO as opposed to its obstruction (odds ratio, 4.73; 95% confidence interval, 1.74-12.9). No malpositioned stents were noted in our cohort.The use of EVO as an outcome marker of iStent surgery is physiologically plausible. The proposed EVO grading scale is simple, comprises few categories, and is easily applicable in an in-office setting. The results of this study suggest this scale could be useful in the assessment of iStent functionality and encourage its further investigation in prospective studies.