PURPOSE: To report 5-year postoperative safety data of iStent inject, including overall stability, endothelial cell density (ECD), and endothelial cell loss (ECL) in patients with mild-to-moderate primary open-angle glaucoma (POAG). DESIGN: 5-year follow-up safety study of the prospective, randomized, single-masked, concurrently controlled, multicenter iStent inject pivotal trial. METHODS: In this 5-year follow-up safety study of the 2-year iStent inject pivotal randomized controlled trial, patients receiving iStent inject placement and phacoemulsification or phacoemulsification alone were studied for the incidence of clinically relevant complications associated with iStent inject placement and stability. Corneal endothelial endpoints were mean change in ECD from screening and proportion of patients with >30% ECL from screening, from analysis of central specular endothelial images by a central image analysis reading center at several time points through 60 months postoperatively. RESULTS: Of the 505 original randomized patients, 227 elected to participate (iStent inject and phacoemulsification group, n = 178; phacoemulsification-alone control group, n = 49). No specific device-related adverse events or complications were reported through month 60. No significant differences were observed in mean ECD, mean percentage change in ECD, or proportion of eyes with >30% ECL between the iStent inject and control groups at any time point; mean percentage decrease in ECD at 60 months was 14.3% ± 13.4% in the iStent inject group and 14.8% ± 10.3% in the control group (P = .8112). The annualized rate of ECD change from 3 to 60 months was neither clinically nor statistically significant between groups. CONCLUSIONS: Implantation of iStent inject during phacoemulsification in patients with mild-to-moderate POAG did not produce any device-related complications or ECD safety concerns compared to phacoemulsification alone through 60 months.