By estimating cup/disc ratios in fellow eyes it has been assumed that nonarteritic anterior ischemic optic neuropathy (AION) occurs more often in small optic nerve heads. Correcting the photographic magnification we used absolute size units to measure 33 affected and 25 fellow optic discs with nonarteritic AION and 7 affected and 7 fellow optic nerve heads with arteritic AION. The affected and fellow discs with nonarteritic AION (2.37 + -0.29 mm2 and 2.31 + -0.31 mm2) were significantly (p less than 0.001, Mann-Whitney-test) smaller than 457 normal optic nerve heads (2.69 + -0.70 mm2). They were significantly (p less than 0.001; Mann-Whitney-test) larger than optic nerve heads with pseudopapilledema or drusen. Affected and fellow optic nerve heads with arteritic AION were not significantly different in size from normal discs but significantly (p less than 0.005) larger than the discs affected by nonarteritic neuropathy. There were no significant form differences between the pathologic and normal discs. Optic disc morphometry can be helpful in the differentiation of nonarteritic and arteritic AION: Nonarteritic AION occurs more often in small optic discs, arteritic AION is more often in normal sized optic nerve heads.