PURPOSE: To report the baseline characteristics of participants enrolled in TAGS (Treatment of Advanced Glaucoma Study). DESIGN: Pragmatic randomized control trial (RCT). METHODS: Participants with newly diagnosed advanced glaucoma in at least 1 eye were recruited. Participants were patients with open angle glaucoma presenting with advanced glaucoma in at least 1 eye as defined by the Hodapp-Parrish-Anderson (HPA) criteria for severe defect. Participants were randomly allocated to receive either primary augmented trabeculectomy or primary medical management. When both eyes were eligible, the same intervention was undertaken in both eyes, and the index eye for analysis was the eye with the less severe visual field mean defect (MD). Main outcome measurements were visual field profile, defined by the HPA classification; clinical characteristics; quality of life, as measured by the National Eye Institute Visual Function Questionnaire 25 (VFQ-25), the EuroQual-5 Dimension (EQ-5D 5L), Health Utility Index-3 (HUI-3), and the Glaucoma Profile Instrument (GPI). RESULTS: A total of 453 patients were recruited. The mean visual field MD was -15.0 dB ± 6.3 in the index eye and -6.2 dB in the non-index eye. Of index eyes (HPA "severe" classification) at baseline, more than 70% of participants had a MD <-12.00 dB, and nearly 90% had more than 20 points defective at the 1% level. The mean LogMAR visual acuity of the index eye was 0.2 ± 0.3. CONCLUSIONS: TAGS is the first RCT to compare medical versus surgical treatments for patients presenting with advanced open angle glaucoma in a publicly funded health service. The study will provide clinical, health-related quality of life, and economic outcomes to inform future treatment choices for those presenting with advanced glaucoma.