Purpose: To assess the impact of a masked Endpoint Committee on estimates of the incidence of primary open-angle glaucoma (POAG) treatment efficacy and statistical power of the Ocular Hypertension Treatment Study-Phase 1, 1994-2002 (OHTS-1). Design: Retrospective interrater reliability analysis of endpoint attribution by the Endpoint Committee. Methods: After study closeout, we recalculated estimates of endpoint incidence, treatment efficacy, and statistical power using all-cause endpoints and POAG endpoints. To avoid bias, only the first endpoint per participant is included in this report. Results: The Endpoint Committee reviewed 267 first endpoints from 1636 participants. The Endpoint Committee attributed 58% (155 of 267) of the endpoints to POAG. The incidence of all-cause endpoints vs POAG endpoints was 19.5% and 13.2%, respectively, in the observation group and 13.1% and 5.8%, respectively, in the medication group. Treatment effect for all-cause endpoints was a 33% reduction in risk (relative risk = 0.67, 95% confidence interval [CI] of 0.54-0.84) and a 56% reduction in risk for POAG endpoints (relative risk = 0.44, 95% CI of 0.31-0.61). Post hoc statistical power for detecting treatment effect was 0.94 for all-cause endpoints and 0.99 for POAG endpoints. Conclusion: Endpoint Committee adjudication of endpoints improved POAG incidence estimates, increased statistical power, and increased calculated treatment effect by 23%. An Endpoint Committee should be considered in therapeutic trials when common ocular and systemic comorbidities, other than the target condition, could compromise study results.