Amaurotic mydriasis is characterized by larger than normal pupils in patients with visual loss. In sought to establish whether amaurotic mydriasis can reliably identify different kinds of visual loss and whether this static measurement might prove useful in discerning "balanced" bilateral optic neuropathies where no relative afferent pupillary defect (RAPD) is detected. Patients with binocular pregeniculate visual loss, patients with balanced binocular pregeniculate loss without RAPD, and patients with monocular pregeniculate visual loss had significantly larger pupils than age-matched controls. Although pupils of patients with binocular pregeniculate visual loss (20/50 or better in one or both eyes) were significantly smaller than pupils of age-matched patients with binocular pregeniculate deficits of worse than 20/50 in one or both eyes, no such correlation between Snellen visual acuity and amaurotic mydriasis was found in patients with monocular pregeniculate visual loss. Amaurotic mydriasis is a clinically useful phenomenon that may identify pregeniculate disease in the absence of a RAPD or distinguish pregeniculate from postgeniculate visual loss.