Background/Aims: Both eyes of a same person are not completely independent entities. The purpose of this study was to assess the difference in outcome between the first-operated eye and the fellow-operated eye following bilateral XEN surgery, and to identify potential success predictors for the fellow eye. Methods: This single-site, prospective, nonrandomized interventional study investigated bilateral nonsimultaneous XEN gel stent implantation over 24 months. Its main outcome measure was surgical success, defined as unmedicated intraocular pressure (IOP) <= 15 mm Hg associated with a relative reduction >= 20%. Results: Of 149 enrolled eyes, 74 eyes of 37 patients who underwent bilateral (standalone or combined) XEN implantation, within a mean of 50.5 +/- 74.3 days of each other, were analyzed. Postoperatively, mean medicated IOP decreased from 19.0 +/- 6.6 (first-operated) and 18.2 +/- 5.2 mm Hg (fellow) at baseline (P=0.209) to 13.7 +/- 4.0 (-27.9%;P<0.001; first-operated) and 14.1 +/- 3.6 mm Hg (-22.5%;P<0.001; fellow) after 2 years (P=0.673). Rates of complete success were comparable between first-operated and fellow eyes (P=0.65). At 2 years, among patients whose first-operated eyes were considered a failure, 92.0% of fellow eyes failed (P=0.001). The odd ratio of a fellow eye experiencing complete success were 16.1 (95% confidence interval: 2.5-101.9,P=0.003) if the first operated eye experienced complete success. Conclusions: The present study demonstrated a strong association between first-operated eyes and fellow eyes following XEN gel stent implantation, in terms of surgical outcomes and IOP reduction. In effect, surgical success in the first-operated eye increases the odds of success in the fellow eye by 16-folds.