Some patients with acute zonal occult outer retinopathy (AZOOR) show symptomatic acute visual impairment in one eye only, but electrophysiological abnormalities in both eyes. A 46-year-old female who had noticed paracentral scotoma of her right eye six months previously was referred to our hospital. At initial examination, her best-corrected visual acuity was 0.7 in the right eye and 1.2 in the left. Whereas the full-field rod electroretinogram (ERG) was normal in both eyes, the cone response was nonrecordable in the right eye and showed a significant decrease in amplitude in the left eye. The 30 Hz flicker ERG showed similar results. Multifocal ERG was nonrecordable in the right eye and showed a residual response in only the central part of the left. Fourteen months after the first visit, the patient presented complaining of acute visual acuity loss in the left eye. Visual acuity in her left eye had decreased to 0.01. The multifocal ERG was nonrecordable. Although the patient chose oral prednisolone therapy, only limited recovery was observed in the patient. Even if only the ERG shows changes and there are no other symptoms, ophthalmologists should continue observation in view of the possibility of an acute exacerbation.