The susceptibility of genital herpes to acyclovir (ACV) in immunocompetent women was examined, as was the frequency of ACV-resistant viruses by analyzing 56 clinical isolates in Japan between 1977 and 1996. The mean susceptibilities of herpes simplex virus (HSV) type 1 and type 2 were 0.13+/-0.74 and 0.42+/-0.14 microg/ml, respectively, assessed by the 50% inhibitory concentration of plaque formation. The susceptibility to ACV of clinical isolates did not changed since 1977, and also that of nine pairs of HSV-1 and HSV-2 isolates was not affected by ACV treatment. In order to characterize the change in the virus population, the quantitation of the ACV-resistant virus in 10(4) plaque forming units (PFU) of clinical isolates was adopted. The mean frequencies of ACV-resistant viruses per 10(4) PFU for all strains of HSV-1 and HSV-2 were 0.31+/-0.41 and 9.74+/-14.83, respectively, and were not influenced by ACV treatment. Additionally, the phenotypes of ACV-resistance were not influenced by ACV treatment, and more than 90% of ACV-resistant viruses were found to be thymidine kinase-deficient. This study characterized clinical isolates with respect to ACV susceptibility as a population and the quantitative and qualitative characterization of ACV-resistant virus in the virus population of clinical isolates was also studied. The susceptibility of isolates from genital lesions, the frequency of ACV-resistant viruses, and also the phenotypic characterization of ACV-resistant viruses was maintained between 1977 and 1996, even after the introduction of ACV treatment for genital herpes in Japan.