Varicella vaccine in immunocompromised children was clinically evaluated in 575 US and Canadian children with leukemia in remission by the Varicella Vaccine Collaborative Study. Most children had chemotherapy stopped 1 week before and 1 week after immunization. Steroids were stopped for 3 weeks (1 week before to 2 weeks after vaccination). Varicella vaccine was safe, immunogenic, and effective in leukemic children at risk for serious disease or death from chickenpox. The major side effect was mild rash in 50% approximately 1 month after immunization. About 40% of children who developed rash were treated with acyclovir. Vaccine efficacy was judged by the degree of protection after a household exposure to varicella; of 123 exposed children, 17 (14%) developed a mild form of varicella. The vaccine protected completely against severe varicella. Leukemic vaccines were less likely to develop zoster than were comparable children with leukemia who had wild type varicella. Thus, varicella vaccine, administered carefully with close follow-up, is extremely beneficial for leukemic children.