Acyclovir has been shown to shorten the course of chickenpox in otherwise healthy children by 25–33% according to three placebo controlled trials in the United States.1 2 3 All clinical events monitored were favourably affected by acyclovir, with the most important differences being reductions in degree and duration of fever, constitutional illness score, and number of residual lesions present at the 28 day follow up examination. No appreciable adverse events were reported. These studies also identified two subsets of patients particularly suitable for acyclovir therapy: adolescents and children who acquired chickenpox from a family member.