In a large pertussis vaccine efficacy trial in Germany, vaccinees and/or their family members were seen if a cough illness of >14 days was reported. Evidence of recent Bordetella pertussis infection included a positive culture and/or polymerase chain reaction (PCR) and/or significant antibody values in agglutination and/or ELISA assay. From July 1991 through February 1994, 246 adults were evaluated and 64 had evidence of B. pertussis infection; of these, 38% had whooping, 26% had a history of previous pertussis, and 48% were the primary cases in a family. The 64 adult cases suggest an adult attack rate in this population of 133 per 100,000 population per year. Since pertussis has been endemic and epidemic in Germany during the last 2 decades, it would seem likely that few persons would escape B. pertussis infections during childhood. In this regard, none of the serological controls lacked antibody to all four B. pertussis antigens (lymphocytosis-promoting factor, filamentous hemagglutinin, pertactin, and fimbriae-2). Thus, serological evidence of past infection may not indicate protection, and the widely held belief that individuals who have had infections with B. pertussis have lifelong clinical immunity to this disease is probably wrong.