Live attenuated varicella vaccine is safe and effective in preventing chickenpox. The best immune responses occur in healthy children. Leukemic children have a 50% incidence of mild-to-moderate adverse effects but have a high degree of protection once immune reactions to varicella-zoster virus (VZV) have developed. Adult vaccinees have a lower degree of protection (70%) than children. Vaccinees who develop breakthrough varicella usually have a modified infection. Another significant advantage of vaccination is that in leukemic children it leads to a lower incidence of zoster than after natural chickenpox. It is possible to differentiate between vaccine-type and wild-type VZV using a combination of polymerase chain reaction and restriction endonuclease digestion. A new assay for antibodies to VZV measured by latex agglutination reveals that 8-10 years after vaccination antibodies are detectable in greater than 90% of leukemic children who have not had breakthrough varicella.