Hypobaric hypoxia, the major environmental factor at high altitudes, has been observed to induce pupil miosis and widening of the anterior chamber angle. This environment may be safe for individuals with narrow angle and deserves further study.This study aimed to quantify anterior chamber biometric parameters before and after acute short-term, effortless exposure to hypobaric hypoxia (HH) in healthy lowlanders using swept-source anterior segment optical coherence tomography (SS AS-OCT).This prospective study included 25 healthy young lowlanders (50 eyes) who underwent SS AS-OCT measurements and intraocular pressure (IOP) assessments under baseline sea-level conditions (T1).They were then passively exposed to simulated 4000 m above sea level for 3 hours and underwent Acute mountain sickness (AMS) symptoms evaluation and IOP measurement after 2-hours exposure to HH (T2).Repeat SS AS-OCT measurements and IOP assessments were taken within 15 minutes after leaving the hypobaric chamber (T3). Anterior segment parameters including anterior chamber depth (ACD),lens vault (LV),angle opening distance (AOD500), trabecular-iris space area (TISA500), angle recess area (ARA500) at 500 μm from the scleral spur, iris curvature (IC), iris volume (IV), pupil diameter (PD), and central corneal thickness (CCT) were obtained through SS AS-OCT. These repeated measurements were compared using linear mixed model analysis.In comparison to sea level, both IOP (16.4±3.4 vs. 14.9±2.4 mm Hg, P=0.029) and PD (5.36±0.77 vs. 4.78±0.89 mm, P=0.001) significantly decreased after exposure to HH. Significant post-HH changes (Mean difference (95% CI)) were observed in AOD500 (0.129 (0.006, 0.252), P=0.04), TISA500 (0.059 (0.008, 0.11), P=0.025), ARA500 (0.074 (0.008, 0.141), P=0.029), IV (1.623 (0.092, 3.154), P=0.038), and IC (-0.073 (-0.146, 0.001), P=0.047), while CCT, ACD, and LV remained stable. After adjusting for age, post-HH variations in AOD500 (Beta=0.553, 95% CI: 0.001, 1.105, P=0.048) and TISA500 (Beta=0.256, 95% CI: 0.02, 0.492, P=0.034) were associated with decreased IC but were not related to lowered arterial oxygen pressure or IV increase per millimeter of pupil miosis (IV/PD). These differences in anterior segment parameters were neither correlated with differences in IOP nor AMS.After short-term, effortless exposure to hypobaric hypoxia, pupil miosis occurred with widening of the anterior chamber angle and decreased IC. These changes in anterior chamber angle parameters were associated with decreased IC but did not correlate with the post-hypobaric variations in IV/PD, IOP, or AMS.Copyright © 2024 The Author(s). Published by Wolters Kluwer Health, Inc.