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To assess the agreement among clinical, Heidelberg Retina Tomograph (HRT), and optic disc photography in the optic disc diameter measurement.Prospective, observational study of 55 eyes of 55 subjects who underwent horizontal and vertical optic disc diameter measurements with +90 diopter lens on slit-lamp biomicroscopy (clinical) and on HRT and optic disc photography. Measurements were compared and agreement among the methods was assessed using intraclass correlation coefficients and Bland and Altman plots.Mean vertical diameter measured by clinical (1.93±0.28 mm) and on disc photography (1.9±0.22 mm) were comparable (P=0.812) and both were significantly greater (P<0.0001) than that measured on HRT (1.69±0.20 mm). Mean horizontal diameter by clinical (1.82±0.19 mm) and on disc photography (1.79±0.22 mm) were comparable (P=0.789) and both were significantly greater (P<0.01) than that measured on HRT (1.66±0.16 mm). The 95% limits of agreement on Bland and Altman plots ranged from 0.54 mm (clinical and disc photography) to 0.93 mm (clinical and HRT) for vertical diameter measurements and from 0.51 mm (HRT and disc photography) to 0.63 mm (clinical and HRT) for horizontal diameter measurements. Proportional bias was detected in the agreement between clinical and HRT as well as clinical and disc photography in vertical disc diameter measurement.Agreement between clinical and disc photography for optic disc diameter is better than that between either of these 2 methods and HRT. With the increasing disc size, there is an increasing disagreement in the vertical disc diameter measurements between the clinical and HRT as well as clinical and disc photography.