It was previously reported that a live varicella vaccine (Oka strain) has been developed and that the immediate vaccination of hospitalized children was effective for prevention of spread of varicella in a ward. Six to nine months later, there were four separate episodes of varicella and zoster in the same ward. Eighteen children (11 with nephrotic syndrome, 6 with nephritis, and 1 with hepatitis) with no history of varicella were inoculated with a live vaccine before or immediately after admittance or occurence of the varicella and zoster cases. Twelve of them had been receiving steroid therapy and 15 of the 18 were found to be seronegative by complement fixation and neutralization tests before the vaccination. All of them became seropositive after vaccination without any clinical symptoms. The longest period between vaccination and exposure was nine months. None of the vaccinees exhibited varicella symptoms after exposure. Serological follow-up of ten vaccinated children was done, and booster responses were observed in some of them after exposure. These results suggest that the live vaccine affords immunity to the recipients. If hospitalized children are vaccinated before or immediately after exposure, isolation of the patient is unnecessary.