A live varicella vaccine (Oka strain) was given to susceptible household contacts of varicella to test the protective efficacy of the vaccination. Twenty-six contacts of 21 families were vaccinated, usually within three days after onset of the index cases. None of the vaccinated children developed clinical symptoms of varicella. Eighteen of 24 sera obtained before vaccination were found to be seronegative by complement fixation and neutralization tests. Seroconversion was observed in all of the 18. On the other hand, all of 19 unvaccinated contacts in the 15 families, who served as controls, showed typical manifestations of varicella from 10 to 33 days after onset of the index varicella cases. In three families, where only one sibling contact received vaccine and the other was an unvaccinated control, none of the vaccinated children showed any clinical symptoms while unvaccinated controls exhibited typical varicella symptoms 10 to 14 days after the onset of the index cases. These results indicate that varicella is prevented in household contacts by vaccination within three days following exposure.