Retinal photoreceptor dysfunction is an uncommon and often unrecognized cause of acute visual loss. Acute zonal occult outer retinopathy (AZOOR) has been reported to cause cone and rod dysfunction. Patients with AZOOR may present with normal visual acuity, normal fluorescein angiography, and a normal fundus examination despite severe loss of visual field. A healthy young white female presented with acute, unilateral loss of visual field and an afferent pupillary defect, but normal visual acuity, color vision, fundus examination, and fluorescein angiogram. A pattern visual evoked potential was normal, but an electroretinogram showed a unilateral peripheral photoreceptor dysfunction consistent with the diagnosis of AZOOR. Ophthalmologists should be aware of the diagnosis of AZOOR and should consider an ERG in the evaluation of any patient with unexplained visual field loss even in the presence of normal visual acuity, color vision, fluorescein angiography, or retinal examination.