A prominent increase of notified cases of pertussis was observed during discontinuation of pertussis immunization in Aichi Prefecture from 1975 to 1979. The patients were principally unimmunized children. After reintroduction of pertussis vaccine the number of cases decreased gradually and the decline has continued after the introduction of acellular pertussis vaccine. A retrospective survey of immunized schoolchildren also showed efficacy of the vaccine. To confirm the efficacy of acellular vaccine a prospective case contact study was conducted. Of 35 unimmunized children 29 developed clinically diagnosed pertussis, and of 52 children who received two or more doses of the vaccine only two developed disease after the household exposure. There were no serious systemic adverse reactions including high fever or encephalopathy. Severe local reaction occurred in 2.65/100,000 recipients of a third and fourth dose of the acellular vaccine.