A study of the effects of oral acyclovir (200 mg), administered five times per day for 5 days in 210 patients who cultured positive for herpes labialis, is made. A total of 149 patients were followed through three episodes each of herpes labialis while taking a placebo or acyclovir. Patients were evaluated for several clinical parameters, including the loss of lesion crust and reduction of the size of the area of the lesion between day 1 and day 5. Acyclovir showed a significant antiviral effect. Results show that oral acyclovir can favorably affect some parameters, but that higher doses or a "loading dose" could improve its efficacy.