Disagreement exists as to whether the depth of anisometropic amblyopia correlates with the degree of anisometropia. We reviewed the charts of 303 consecutive patients with the diagnosis of amblyopia. Thirty-five patients with untreated anisometropia without strabismus were identified. The best-corrected visual acuities, manifest refractions, and autorefractions were recorded. The differences in spherical equivalent, root mean square, and an index of anisometropia, which we defined, were calculated. The depth of amblyopia was strongly correlated to all three measures of anisometropia for both hyperopic and myopic individuals. Our new index of anisometropia was an excellent predictor of the depth of amblyopia.