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) = 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