To evaluate the role of adults in the transmission of pertussis during an epidemic, persons presenting with unexplained cough to ambulatory care clinics were evaluated for evidence of pertussis infection. Nasopharyngeal specimens for culture and serum samples for IgG and IgA antibodies to filamentous hemagglutinin and pertussis toxin antigens of Bordetella pertussis were obtained. Thirty-eight adults were enrolled in the study; 10 (26%) had serologic evidence of B. pertussis infection. Clinical findings were not significantly different among persons with and without evidence of pertussis infection. Pertussis should be considered in the differential diagnosis of persistent cough in all age groups. Future use of new acellular pertussis vaccines in adults may substantially impact the control of the infection.