To evaluate Bordetella pertussis as a cause of persistent cough in adults, we examined 201 patients who had a cough for 2-12 weeks and no pulmonary disease. We obtained the following at presentation: medical history, chest radiograph, respiratory function measurement, nasopharyngeal aspirate for polymerase chain reaction (PCR), nasopharyngeal swab specimen for culture, and a blood sample (acute serum). Four weeks later a second blood sample (convalescent serum) was obtained. Control sera were obtained from 164 age-matched healthy blood donors with no history of cough during the previous 12 weeks. Four patients were B. pertussis culture-positive; 11 (including the culture-positive patients) were B. pertussis PCR-positive; and 33, including 10 of the 11 PCR-positive patients, had serological evidence of recent B. pertussis infection. Pertussis-positive and -negative patients could not be discriminated by a history of cough. We conclude that B. pertussis infection is a common cause of persistent cough in adults. This is of concern, because these patients may be B. pertussis reservoirs from which transmission may occur to infants, in whom the disease can be devastating.