To assess the effectiveness of topical ocular hypotensive medications in patients with open-angle glaucoma and to identify factors associated with postwashout intraocular pressure (IOP) elevation. Secondary analysis of prerandomization data from 2 prospective, multicenter, randomized clinical trials. Fourteen hundred subjects (1,400 eyes, 781 from the HORIZON study of the Hydrus micro-stent and 619 from the COMPASS study of the Cypass micro-stent) with primary open-angle glaucoma who were using 0-4 classes of topical IOP-lowering medication underwent Goldmann applanation tonometry before and after a protocol-defined washout period. The mean (standard deviation) age was 70.7 (8.0) years and 55.6% were female. The change in IOP following washout for patients using 0 (n = 100), 1 (n = 705), 2 (n = 355), 3 (n = 214), or 4 (n = 26) medications was 0.2 (2.8), 5.7 (3.3), 6.9 (3.7), 8.8 (5.0), and 9.5 (4.1) mm Hg, respectively (P < .001, Kruskal-Wallis test). Postwashout IOP change was similar between the HORIZON and COMPASS cohorts. No difference in postwashout IOP change was detected among individual prostaglandin analogues in patients on monotherapy. A generalized linear model identified the following factors to be associated with greater IOP rise upon medication washout: greater number of glaucoma medications, higher unmedicated IOP, thinner central corneal thickness (CCT), lack of prior selective laser trabeculoplasty (SLT), and male sex. Cessation of glaucoma medications results in a dose-dependent IOP increase in treated open-angle glaucoma patients. Two independent clinical trial cohorts exhibit similar levels of IOP elevation upon washout, using standardized methodology to estimate real-world medication effectiveness. Thicker CCT and history of SLT may predict reduced response to IOP lowering medications.