BACKGROUND AND PURPOSE: No MR imaging measurement criteria are available for the diagnosis of optic nerve atrophy. We determined a threshold optic nerve area on MR imaging that predicts a clinical diagnosis of optic nerve atrophy and assessed the relationship between optic nerve area and retinal nerve fiber layer thickness measured by optical coherence tomography, an ancillary test used to evaluate optic nerve disorders. MATERIALS AND METHODS: We evaluated 26 patients with suspected optic nerve atrophy (8 with unilateral, 13 with bilateral and 5 with suspected but not demonstrable optic nerve atrophy) who had both orbital MR imaging and optical coherence tomography examinations. Forty-five patients without optic nerve atrophy served as controls. Coronal inversion recovery images were used to measure optic nerve area on MR imaging. Retinal nerve fiber layer thickness was determined by optical coherence tomography. Individual eyes were treated separately; however, bootstrapping was used to account for clustering when appropriate. Correlation coefficients were used to evaluate relationships; receiver operating characteristic curves, to investigate predictive accuracy. RESULTS: There was a significant difference in optic nerve area between patients' affected eyes with optic nerve atrophy (mean, 3.09 ± 1.09 mm), patients' unaffected eyes (mean, 5.27 ± 1.39 mm; = .008), and control eyes (mean, 6.27 ± 2.64 mm; < .001). Optic nerve area ≤ 4.0 mm had a sensitivity of 0.85 and a specificity of 0.83 in predicting the diagnosis of optic nerve atrophy. A significant relationship was found between optic nerve area and retinal nerve fiber layer thickness ( = 0.68, < .001). CONCLUSIONS: MR imaging-measured optic nerve area ≤ 4.0 mm has moderately high sensitivity and specificity for predicting optic nerve atrophy, making it a potential diagnostic tool for radiologists.