Varicella is relatively mild in otherwise normal children, in whom new lesions form for a mean of four days after onset and heal 50 percent of their lesions in eight days. New lesions form in most immunocompromised children for longer than five days and those not treated with antiviral drugs have a 28 percent incidence of pneumonitis and a 7 percent mortality rate. Untreated immunocompromised adults with herpes zoster shed virus for longer (7.0 days) than otherwise normal adults (5.3 days). Herpes zoster is much more likely to disseminate cutaneously in immunocompromised than in immunocompetent hosts. Visceral dissemination, which is a rare event in immunocompetent patients, occurred in 8 percent of prospectively followed untreated immunocompromised hosts with herpes zoster. Acyclovir has been found to be superior to vidarabine for treatment of both chickenpox and herpes zoster. Whether or not steroids should be used to treat herpes zoster remains controversial. Concerns about the use of intravenous acyclovir include the side effects of renal and central nervous system dysfunction and the possibility of emergence of resistant viral strains. None of these concerns has proved to be an impediment to successful treatment of immunocompromised patients. The major future challenge is to find an optimal way to treat varicella zoster virus infections with oral formulations of acyclovir or its congeners.