The RVD measured from the digital fundus images increased significantly by 8.9 μm (SD 4.3, T = 8.865, P < 0.001) at altitude, but there was no significant difference between intervention groups (F = 1.644, P = 0.219). Retinal artery diameter was also significantly raised with altitude, by 7.0 μm (SD = 3.4, T = 7.050, P < 0.001), and was similarly unaffected by acetazolamide (F = 0.439, P = 0.518). There was a significant positive correlation between the presence of HAH and both RVD (T = 4.953, P = 0.001) and retinal artery diameter (T = 2.865, P = 0.015). However, this did not extend to AMS, where there was no significant correlation between AMS scores and either retinal vein (T = 0.275, P = 0.788) or retinal artery (T = 0.536, P = 0.610) changes. Diameters measured using SLO images were highly correlated with those from the digital fundus camera for both retinal veins (r = 0.772, P < 0.001) and retinal arteries (r = 0.813, P < 0.001), with identical statistical conclusions regarding altitude, HAH and AMS. To reduce the chance that these changes were due to an optical effect due to altitude-related pressure changes a third structure, the optic disc diameter, was also compared. This showed an extremely small and nonsignificant decrease (T = 0.583, P = 0.568) in the mean disc diameter, from 1514.6 to 1513.7 μm.