Herpes Zoster (HZ) is estimated to affect between 600,000 to 1 million people annually in the United States. The incidence and severity of HZ, as well as the frequency and severity of its complications, increase markedly with age. Antiviral therapy has modest impact on the acute phase of HZ. However, it does not appear to prevent post herpetic neuralgia, the most common complication of HZ. During the past decade studies have revealed a close temporal relation between the age-related increase in the incidence and severity of HZ and an age-related decline in cell mediated immunity to varicella-zoster virus (VZV). The administration of a live attenuated varicella-zoster vaccine to older adults results in a marked and long lasting increase in VZV-specific cell-mediated immunity. We hypothesize that by boosting VZV-specific cellular immune responses, the incidence and severity of HZ and its complications can be reduced. Named the Shingles Prevention Study (SPS), this major clinical research study is a five-year, nationwide trial being conducted at 22 study sites, and enrolled a total of 38,546 volunteers. Of these individuals, 1,741 were vaccinated at the NIH Clinical Center. The enrollment period lasted from November 1998 to July 2001. At our site, the study received IRB approval on 5/3/99 and the first patient was enrolled on 6/15/99. Reactions to the vaccine were monitored passively by phone and actively in a 6000-subject subset. Surveillance of the enrollees is performed by a monthly automatic phone system. Patients reporting a zoster-like illness (either into this system or by calling directly) are examined and tested for VZV. We have evaluated and followed 49 cases of herpes zoster at our site. To this point, the Data Safety Monitoring Board reviewing of the severe adverse events and the suspected cases of herpes zoster distribution by treatment group (vaccine vs. placebo) has concluded that there has been no suggestion of any vaccine-related safety problems.