INTRODUCTION: This study aimed to evaluate the characteristics of the filtering bleb in failed late trabeculectomy and the agreement between glaucoma specialist assessment and anterior segment optical coherence tomography (AS OCT) system assessment in terms of the presence of subconjunctival fluid in the filtering bleb. METHODS: Patients with a diagnosis of glaucoma who were scheduled to undergo trabeculectomy in at least one eye and with uncontrolled intraocular pressure (IOP) were enrolled. All participants underwent a complete ophthalmological examination. The presence of fluid under the filtering bleb was first evaluated by a glaucoma specialist using biomicroscopy evaluation and then using the OCT 1000 anterior segment OCT Version 126.96.36.199 (Carl Zeiss Meditec, Dublin, California, USA) system. A Kappa statistical test was used to evaluate the agreement between AS OCT and the examiner. The correlation between conjunctiva and tenon thickness and clinical parameters was also assessed. RESULTS: Forty eyes of 40 patients were evaluated in this study. The ages ranged from 21-86 years, with a mean of 66.55±12.33. The average IOP was 21.20±4.44 mmHg (range 14-38 mmHg) in the entire group. The mean thickness of the conjunctiva and tenon was 302.03±406.76 µm (range 251-1616 µm). There was a significant negative correlation between the mean thickness of the conjunctiva and tenon and IOP (p=0.045; confidence interval=-0.558,0.024). Additionally, there was a significant negative correlation between the mean thickness of the conjunctiva and tenon and the number of medications used at baseline (p=0.043; confidence interval=-0.538,0.051). There was significant negative correlation between the horizontal measurement of the bleb and the use of glaucoma medications (p=0.017; confidence interval=-0.560,0.004). A total of 26 patients were determined to have fluid by the examiner (glaucoma expert), and the presence of fluid in AS was confirmed in 19 patients by anterior segment OCT. Of the 14 patients who were determined to have an absence of fluid, this was confirmed by AS OCT in seven patients (Kappa=0.231; agreement of 65.00%). CONCLUSION: There is fair agreement between glaucoma specialist assessment and AS OCT assessment in terms of the presence of fluid in trabeculectomy. Our findings highlight the importance of AS OCT in some patients before deciding upon a new intervention.