Acute retinal necrosis (ARN) is characterized by unilateral or bilateral severe vaso-occlusive retinitis. In a large number of cases the patients become blind in the affected eyes as a direct result of the acute inflammation or the subsequent retinal detachment. The etiology is poorly understood; herpesvirus-induced vasculitis is presumed and has been demonstrated by some authors. However, the author's own investigations in seven patients show that even vitreous biopsy reveals a herpetic origin only in a minority of cases. Intravenous treatment with acyclovir and steroids prevented progression of the retinitis but did not reduce the risk of retinal detachment.