A 48-year-old male diagnosed with right-onset herpes zoster ophthalmicus developed visual acuity loss in the left eye during the following 3 weeks. Visually evoked cortical potential recordings revealed a marked increase in P100 latency and a marked decrease in its amplitude in both eyes. Pattern electroretinography suggested diffuse pathology with reduced positive and negative components. A possible transsynaptic or intraneural spread of the varicella-zoster virus in the optic nerve might be responsible for this unexplained contralateral loss of visual acuity.