In winter 2001, an outbreak of pertussis involving an estimated 75 people occurred among soldiers serving in an infantry regiment of the Israeli Defense Forces (IDF). Nasopharyngeal swabs were obtained from patients and contacts for culture and PCR. Serum samples were obtained and assayed by ELISA for the presence of IgA, IgM and IgG antibodies to a lysate antigen of Bordetella pertussis. The calculated attack rate was 21% based on clinical signs alone (cough lasting 30 days or longer) and 9·5% based on clinical signs with laboratory confirmation (by PCR, IgA or IgM). A high carriage rate was observed; 20% of the asymptomatic and previously symptomatic subjects were PCR-positive for B. pertussis. These findings emphasize the importance of B. pertussis as a causative agent of epidemic respiratory infections in young adults and reveal the occurrence of a significant proportion of pertussis transient carriers during an outbreak of the disease.