The prevalence and molecular characteristics of isolates from 173 immunocompetent patients with herpetic keratitis (HK) who were infected with acyclovir (ACV)-resistant (ACV(R)) corneal herpes simplex virus (HSV)-1 was determined. Isolates from 11 (6.4%) of the patients were ACV(R), and 9 of these 11 patients were refractory to therapy with ACV; the ACV(R) isolates from 5 and 1 of these 9 patients were cross-resistant to gancyclovir and to both gancyclovir and foscarnet, respectively. Of the 11 ACV(R) isolates, 10 had, in the thymidine kinase gene, mutations that presumably conferred the ACV(R) phenotype. These data demonstrate a relatively high prevalence of corneal HSV-1 ACV(R) isolates in patients with HK, which emphasizes the need to monitor for ACV susceptibility in patients with HK who are refractory to therapy with ACV.