A live varicella vaccine (Oka strain) has been developed and used since 1974 in normal and diseased children, particularly those at high risk. Children with acute leukaemia were usually vaccinated while in remission when showing a normal cell-mediated immunity as assessed with phytohaemagglutinin (PHA) or other reagents, and during suspension of all anticancer therapy, except 6-mercaptopurine from 1 week before to 1 week after vaccination. While clinical reactions were observed in only 40 out of 263 (15.2%) of these patients, they were noted in 30 out of 72 (41.7%) children immunized without suspension of chemotherapy. Symptoms were mostly mild; only a few cases of the latter group with T-cell leukaemia or malignant lymphoma developed severe symptoms. An immune response was observed in most vaccinees, but some (11%) developed clinical symptoms after exposure to natural varicella due to immunodepression caused by continuing anticancer chemotherapy. In these cases, revaccination seems advisable. The incidence and severity of zoster in vaccinated acute leukaemic children were less than in those with natural infection. Satisfactory immune responses with few concomitant clinical reactions were observed in approximately 1,500 vaccinees having nonmalignant diseases and in about 4,000 normal children. A 7-10-year follow-up study revealed that the vaccine had conferred solid immunity on the children. These results indicate that live varicella vaccine of the Oka strain is useful in preventing varicella in high-risk as well as in normal children.