Summary: The topical application of corticosteroids to the eye of man or rabbit infected with Herpes simplex virus markedly aggravates the keratitis and may lead to loss of vision. In the experimental herpetic keratitis of rabbits this deleterious effect could not be attributed to enhancement of viral proliferation. Virus titers were similar in steroid-treated or untreated rabbit corneas during the entire course of infection. It is suggested that corticosteroids may exert their harmful effect in this infection by interfering with normal reparative processes and with the structural integrity of the cornea.