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The purpose of this study was to investigate the diurnal variation in peripapillary and macular vessel density (VD) measurements using optical coherence tomographic angiography (OCT-A) and its correlation to intraocular pressure (IOP) changes in glaucoma patients.Prospective, observational cross-sectional study including 37 patients (74 eyes; age, 63.8±12.9 y) with open-angle glaucoma. OCT-A imaging and IOP measurements were performed at 08:00, 11:00, 14:00, and 16:00 timepoints on a single day. At each timepoint, 2 scan protocols were used to generate 3-dimensional en face OCT angiograms: 4.5×4.5-mm scan centered on the optic nerve head and 6×6-mm scan centered on the fovea. For each scan mode, the “radial peripapillary capillary” segment, composed of the vasculature of the retinal nerve fiber layer and ganglion cell layer, was calculated. Two trained readers reviewed OCT-A image quality. Only scans with signal strength intensity (SSI) higher than 46 and without image artifacts interfering with measurements were included. Variation in VD measurements assessed using analysis of variance (ANOVA) and the association between VD and IOP changes assessed using linear mixed modeling methods.The optic nerve head and peripapillary VD measurements at 14:00 and 16:00 timepoints were greater than the measurements at 08:00 and 11:00 timepoints. The 14:00 and 16:00 VD measurements were statistically significantly greater (P<0.05) than the 08:00 measurements for the whole en face (50.1% and 50.1% vs. 49.4%), inside disc (50.6% and 50.5% vs. 49.6%), and average peripapillary (58.2% and 58.5% vs. 57.5%) VDs. The macular VD measurements at the 14:00 timepoint were greater than the measurements at 08:00 and 11:00 timepoints. Changes in VD were significantly associated with changes in SSI but not IOP.Diurnal changes in OCT-A-measured VD in glaucoma patients were small and clinically insignificant. These changes were not associated with IOP changes.