Acanthamoeba keratitis is uncommon and rarely reported in Korea. It has been reported in world literature as a very severe, progressive necrotizing stromal keratitis due to a non-parasitic free-living amoeba. It is frequently associated with minimal corneal trauma especially from contact lens but sometimes occurs in patients without any past history. We report a case of acanthamoeba keratitis without a specific past history in a 42-year-old man. Light and electron microscopy demonstrated severe stromal keratitis with numerous thick-walled cysts, 10~15 m in diameter, scattered in the superficial and deep stroma. Because this keratitis is most often mistaken for fungal, bacterial or herpetic keratitis, early confirmatory diagnosis by direct smear, biopsy or culture is essentially required for the prevention of visual loss or devastating eyeball loss.