STUDY OBJECTIVE: To determine whether orally administered acyclovir is of therapeutic benefit for varicella in otherwise healthy adolescents, and to compare the severity of the disease in adolescents with that in younger children. DESIGN: Multicenter, randomized, placebo-controlled, double-blind trial. SETTING: Patients' homes and university hospital clinics. PATIENTS: Sixty-eight adolescents between 13 and 18 years of age with varicella entered the study. Of the 62 adolescents with laboratory-confirmed varicella who were included in the final analysis, 31 received acyclovir and 31 received placebo. INTERVENTIONS: Placebo or an 800 mg acyclovir tablet was given orally four times daily for 5 days, beginning within 24 hours of onset of rash. MEASUREMENTS AND MAIN RESULTS: Acyclovir recipients had significant reductions in times to cessation of new lesion formation (p less than 0.001), maximum number of lesions (p = 0.019), and defervescence (p = 0.045). Mean constitutional illness score was significantly reduced on day 4 (0.5 vs 1.5, p = 0.05), as was the mean number of residual hypopigmented lesions present on 28-day follow-up examination (22.7 vs 92.7, p = 0.018). Two complications, both bacterial superinfections, occurred in placebo recipients. Adverse experiences and varicella-zoster virus antibody titers measured 28 days after enrollment were similar in both treatment groups. Comparison of placebo recipients with children 2 to 12 years of age participating in a companion study indicated that varicella is more severe in adolescents: mean maximum total lesions (421 vs 347, p = 0.003), mean maximum constitutional illness score (3.1 vs 2.2, p = 0.032), and mean number of residual lesions (92.7 vs 33.2, p = 0.01) were all greater in the adolescent population. CONCLUSIONS: Oral acyclovir therapy is safe and effective for treatment of varicella in otherwise healthy adolescents; this may be an appropriate subgroup for treatment with antiviral drugs because the disease is more severe in them than in younger children.