OBJECTIVE : Acute retinal necrosis (ARN) is a severe necrotizing retinitis caused by varicella-zoster virus or herpes simplex virus (HSV) that often results in blindness. Occasionally, HSV-caused ARN develops after herpes simplex encephalitis (HSE). It remains unknown, however, when and how often ARN develops after HSE. To investigate the actual conditions of patients with ARN following HSE and the interval period between the prior HSE and the onset of ARN, a retrospective nationwide survey of the Japanese population was performed. METHODS : Retrospective. METHODS : Questionnaires were sent out to the neurology and ophthalmology departments of teaching hospitals in Japan. They inquired about HSE patients in neurology departments and ARN patients diagnosed with HSV in ophthalmology departments. The proportion of the HSV-ARN patients with a history of HSE and their interval periods were obtained from the questionnaires returned by the ophthalmology departments. RESULTS : Neurology departments of 324 hospitals responded to the questionnaires (response proportion: 40.9%), and 53 HSE cases were reported. Ophthalmology departments of 535 hospitals responded the questionnaires (response proportion: 54.3%), and 67 HSV-ARN cases were reported. Among the 67 HSV-ARN cases, 16 (23.9%) had histories of prior HSE. Although the interval periods from the prior HSE to the onset of HSV-ARN varied among cases, nearly half developed HSV-ARN within 2 years. CONCLUSIONS : This nationwide survey of the Japanese population showed that HSV-ARN develops after HSE in higher frequency within 2 years. Neurologists and ophthalmologists should be aware that HSE survivors have a risk of contracting HSV-ARN.