Purpose To report 3-year outcomes of the HORIZON study comparing cataract surgery (CS) with Hydrus Microstent (Ivantis, Inc) implantation versus CS alone. Design Multicenter randomized clinical trial. Participants Five hundred fifty-six eyes from 556 patients with cataract and primary open-angle glaucoma (POAG) treated with 1 or more glaucoma medication, washed out diurnal intraocular pressure (IOP) of 22 to 34 mmHg, and no prior incisional glaucoma surgery. Methods After phacoemulsification, eyes were randomized 2:1 to receive a Hydrus Microstent or no stent. Follow-up included comprehensive eye examinations through 3 years. Main Outcome Measures Outcome measures included IOP, medical therapy, reoperation rates, visual acuity, adverse events, and changes in corneal endothelial cell counts. Results Three hundred sixty-nine eyes were randomized to microstent treatment and 187 to CS only. Preoperative IOP, medication use, washed-out diurnal IOP, and glaucoma severity did not differ between the two treatment groups. At 3 years, IOP was 16.7 ± 3.1 mmHg in the microstent group and 17.0 ± 3.4 mmHg in the CS group (P = 0.85). The number of glaucoma medications was 0.4 ± 0.8 in the microstent group and 0.8 ± 1.0 in the CS group (P Conclusions Combined CS 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 filtration surgery than CS alone at 3 years.