A 50-year-old man with multiple sclerosis developed unilateral acute retinal necrosis following a long-term systemic corticosteroid administration for treatment of an attack of paraparesis. The patient was treated with systemic acyclovir, aspirin, oral steroids and topical cyclopentolate and prednisolone acetate for almost 3 months. Pars plana vitrectomy and 360° endolaser photocoagulation posterior to necrotic retinal areas were performed a week after the initial diagnosis. Varicella zoster DNA was confirmed by PCR analysis in the vitreous sample. Two months later, pars plana vitrectomy with silicone oil injection was performed successfully to treat complicated retinal detachment. Careful peripheral fundus examination is essential when acute unexplained visual loss is noted in patients receiving systemic corticosteroids to exclude acute retinal necrosis syndrome.