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OBJECTIVE To report 3-year outcomes of the HORIZON study comparing cataract surgery with Hydrus Microstent versus cataract surgery alone. DESIGN Multicenter randomized clinical trial. PARTICIPANTS Five hundred fifty-six eyes from 556 patients with cataract and POAG treated with ≥ 1 glaucoma medication, washed out diurnal intraocular pressure (DIOP) 22-34 mmHg and no prior incisional glaucoma surgery. METHODS Following phacoemulsification, eyes were randomized 2:1 to receive a Hydrus® Microstent (Ivantis, Inc.) or no stent. Follow-up included comprehensive eye examinations through 3 years postoperatively. MAIN OUTCOME MEASURES Outcome measures included IOP, medical therapy, reoperation rates, visual acuity, adverse events, and changes in corneal endothelial cell counts. RESULTS 369 eyes were randomized to microstent treatment (HMS) and 187 to cataract surgery only (CS). Preoperative IOP, medication usage, washed out DIOP, and glaucoma severity did not differ between the two treatment groups. At 3 years, IOP was 16.7 ± 3.1 in the HMS group and 17.0 ± 3.4 in the CS group (p=0.85). The number of glaucoma medications was 0.4 ± 0.8 in the HMS group and 0.8 ± 1.0 in the CS group (p<0.001), and 73% of eyes in the HMS group were medication free compared to 48% in the CS group (p<0.001). The HMS group had a higher proportion of eyes with IOP ≤18 mmHg without medications compared to CS (56.2% vs. 34.6%, p<0.001) as well as IOP reduction of at least 20, 30 or 40 percent compared to CS alone. The cumulative probability of incisional glaucoma surgery was lower in the HMS group (0.6% vs. 3.9%, hazard ratio = 0.156, 95% CI 0.031 to 0.773, p=0.020). There was no difference in postoperative corneal endothelial cell loss between groups. There were no procedure or device related serious adverse events resulting in vision loss in either group. CONCLUSIONS Combined cataract surgery and microstent placement for mild to moderate POAG is safe, more effective in lowering IOP with fewer medications, and less likely to result in further incisional glaucoma filtrations surgery than cataract surgery alone at 3 years.