To determine the safety and efficacy of varicella vaccine, 17 children with acute lymphoblastic leukemia (ALL) and 2 children with non-Hodgkin lymphoma (NHL) receiving chemotherapy in remission were immunized with live attenuated varicella vaccine (Oka strain). Rash occurred in 7 children 7-53 days (median 24 days) after vaccination. There were 10-80 (median 20) erythematous vesicles and low-grade fever. All children required no specific treatment. There was no spread of varicella to their susceptible siblings. The control group comprised 92 ALL and 25 NHL patients receiving chemotherapy in remission who were nonvaccinated and susceptible to varicella. Over a risk duration of 80 person-years, one child of the vaccinated group developed varicella of mild degree, whereas in a risk duration of 120 person-years, 14 of the control group developed varicella. One patient died, though prompt antiviral therapy, especially acyclovir, was given to all of them. Herpes zoster of mild degree was observed in one child of the vaccinated group 65 days after vaccination. Two children of the control group developed disseminated herpes zoster. With acyclovir therapy, there was no mortality. The incidence of varicella in the vaccinated group was less than that of the control group. The difference is statistically significant (p = .0222), and the side effects of the vaccine were acceptable. Thus we conclude that the vaccine can be safely used in children with ALL or NHL under chemotherapy and can effectively protect such children from varicella.