PURPOSE: To determine the rate of visual field (VF) loss before and after the diagnosis of primary open angle glaucoma (POAG) in the Ocular Hypertension Treatment Study (OHTS). DESIGN: Prespecified analyses of data collected prospectively in a clinical trial with extended follow-up. SETTING AND PARTICIPANTS: Participants who developed POAG during OHTS 1 and 2 (February 1994 to December 2008) constitute an inception cohort. Follow-up data were collected in OHTS 3 (January 2016-April 2019). Analyses were performed from July 2021 to August 2022. INTERVENTION: In OHTS 1 and 2, visual field (VF) tests were performed every 6 months and stereoscopic optic disc photographs were taken every 12 months. These tests were repeated in OHTS 3. MAIN OUTCOMES AND MEASURES: Slopes of mean deviation (MD) were calculated by linear regression for all eyes in OHTS 1 and 2: eyes that did not develop POAG, eyes that developed optic disc POAG only, and eyes that developed VF POAG with/without optic disc POAG. Mean pre- and post-POAG slopes were calculated for eyes with a minimum of 5 VFs for each period. RESULTS: Mean age at diagnosis of POAG was 66.4 ± 9.5 SD years (n = 282 participants), (56%) were male, 61% were White non-Hispanic and 32% were Black not Hispanic by self-report. The post-POAG slope was -0.40 ± 0.64 SD dB/year for all POAG eyes (n = 280 eyes), -0.19 ± 0.4 SD dB/yr. for optic disc POAG only eyes (n = 112 eyes), and -0.54 ± 0.7 SD dB/yr. for VF POAG eyes with or without optic disc POAG (n = 168 eyes). Among the VF POAG eyes, 69 (41%) had post-POAG MD slopes worse than or equal to -0.5 dB/year, 35 (21%) had slopes worse than or equal to -1.0 dB/year, and 9 (5.4%) had slopes worse than or equal to -2.0 dB/year. CONCLUSIONS AND RELEVANCE: Some participants in OHTS had rapid rates of VF loss in one or both eyes despite being followed in a clinical study. This emphasizes that ocular hypertensive patients require careful follow-up, especially those at high risk of developing POAG to ensure early diagnosis and appropriate treatment of POAG.