A double blind, placebo controlled trial of 5% acyclovir cream, applied topically five times a day for five days, was carried out in 49 patients with recurrent herpes labialis. These patients had a total of 74 episodes, 34 of which were treated with the 5% acyclovir cream and 40 with matching placebo. First episodes and all episodes treated with acyclovir cream had significantly shorter times to formation of ulcer or crust and to complete healing (p less than 0.05 for all variables). The duration of all symptoms and proportion of patients developing itching was also reduced by acyclovir cream in first episodes, though the difference was not significant. When the patient started treatment early in the course of a first episode acyclovir cream significantly reduced the percentage of lesions progressing beyond the papular stage (p less than 0.05). Acyclovir cream is well tolerated and effective for the treatment of recurrent herpes labialis.