Objective: To determine the incidence of retinal vein occlusion (RVO) in the Ocular Hypertension Treatment Study (OHTS). Design: Retrospective analysis of data from a randomized clinical trial. Participants: We included 1636 ocular hypertensive participants with a mean follow-up of 9.1 years. Participants in the medication and observation groups were managed according to their original randomization assignment until June 1, 2002. At that time, the observation participants were offered ocular hypotensive treatment. Data to July 1, 2005, are included in this report. Methods: Occurrences of RVO in study participants, categorized as branch, central or hemicentral vein occlusion, were documented. Potential RVO events were identified by a keyword search of Adverse Event Reports, the Optic Disc Reading Center database, Endpoint Committee reviews, and by response to a written request for information sent to each clinical site. To confirm a potential RVO, the complete OHTS chart was reviewed. Statistical analyses included t tests, chi-square tests and Cox proportional hazards models. Main Outcome Measures: Incidence of RVO. Results: Twenty-six RVOs-5 branch, 14 central, and 7 hemicentral RVOs-were confirmed in 23 participants (15 observation and 8 medication). The 10-year cumulative incidence of RVO was 2.1% in the observation group and 1.4% in the medication group (P = 0.14; log-rank test). At baseline, participants who later developed a RVO were significantly older (65.1 vs 55.3 years; P = 0.01), and had greater horizontal cup-to-disc ratios (P = 0.0004). Conclusions: Although the incidence of RVO was higher in the observation group than the medication group, this difference did not attain significance. Consistent with some previous studies, older age and greater cup-to-disc ratio were associated with the development of RVO. Financial Disclosure(s): Proprietary or commercial disclosure may be found after the references.