Background: The prevalence of glaucoma is higher in African American individuals than in white individuals. Objective: To report the safety and efficacy of topical ocular hypotensive medication in delaying or preventing the onset of primary open-angle glaucoma (POAG) among African American participants in the Ocular Hypertension Treatment Study. Methods: Eligibility criteria included age between 40 and 80 years, intraocular pressure between 24 and 32 mm Hg in one eye and between 21 and 32 mm Hg in the other eye, and no evidence of glaucomatous structural or functional damage by standard clinical measures. Participants were randomized to either the observation group or medication group. Of the 1636 participants randomized, 408 were self-identified as African American. Main Outcome Measure: The primary outcome was the development of reproducible visual field abnormality and/or reproducible optic disc deterioration attributed to POAG. Results: Among African American participants, 17 (8.4%) of 203 in the medication group developed POAG during the study (median follow-up, 78 months) compared with 33 (16.1%) of 205 participants in the observation group (hazard ratio, 0.50; 95% confidence interval, 0.28-0.90; P=.02). Conclusion: Topical ocular hypotensive therapy is effective in delaying or preventing the onset of POAG in African American individuals who have ocular hypertension.