We examined the magnification-corrected optic disc size in 54 patients with exfoliative glaucoma, 61 patients with primary open angle glaucoma, and 50 patients with low-tension glaucoma. The mean optic disc area in low-tension glaucoma was statistically significantly larger than that in primary open angle and exfoliative glaucoma. The mean values of eyes with primary open angle and exfoliative glaucoma did not differ significantly from each other. Frequency distribution of the optic disc size showed, however, that all three diagnostic groups differed significantly from each other. Small discs were more frequent in eyes with exfoliative glaucoma, and large discs were more frequent in eyes with low-tension glaucoma. In primary open angle glaucoma, small and large optic discs were found equally frequently. It is possible that in some eyes large optic discs are vulnerable to even low intraocular pressures due to qualitative properties of the extracellular matrix.