Eighty patients who were culture-confirmed positive for recurrent herpes labialis were entered into a clinical trial. Sixty of these completed two episodes each in a randomized, double-blind, placebo-controlled clinical trial of 5 per cent acyclovir in a polyethylene glycol ointment base. The 5 per cent acyclovir ointment failed to show better clinical healing effects than placebo in both documented episodes. There were no serious side effects observed in either group.