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To compare the vessel calibre measurements between optical coherence tomography angiography (OCTA) and colour fundus photography.In this retrospective comparative study, OCTA and colour fundus images of healthy eyes and eyes with optic atrophy were evaluated. The colour fundus image was registered manually using Image J software to the OCTA image of the optic disc. Two independent graders measured the vessel calibre of the widest vein and artery in each peripapillary quadrant on a 3.4 mm diameter circle centred on the optic disc in the same location on both images. The difference in vessel calibre between the two techniques was assessed.A total of 312 vessels from 29 healthy eyes and 20 eyes with atrophic optic discs were included. There was a high level of agreement between graders for measurement of vessel calibre in both colour fundus (intraclass correlation coefficient=0.93, coefficient of variation=0.07) and OCTA images (intraclass correlation coefficient=0.94, coefficient of variation=0.05). The mean vessel calibre in colour fundus images (94.5±23.2 µm) and OCT images (112.2±26.1 µm) was correlated (r=0.8, p<0.001), but the difference was statistically significant (mean difference: 17.6±1.5 µm, p<0.001). This difference was evident for both arteries (mean difference: 18.2±16.3 µm, p<0.001) and veins (mean difference: 15.1±16.2 µm, p<0.001) individually, with a similar magnitude of difference for both vessel types (p=0.08). In addition, the magnitude of difference between imaging modalities was similar in atrophic and healthy discs (17.1±15.9 vs 18.4±15.2 µm, respectively, p=0.4). The difference, however, was significantly higher in vessels with a calibre of ≤94.5 compared with larger vessels (19.3±16.3 vs 15.6±14.4 µm, respectively, p=0.02).Vessel calibre measurements were significantly larger in OCTA images compared with colour fundus photographs, particularly for smaller vessels. These differences may need to be accounted for when using OCTA-derived metrics.