To determine the frequency of pertussis in children <= 16 y who had prolonged cough (>= 14 d), a prospective study was conducted at an outpatient clinic of a paediatric hospital. Nasopharyngeal swabs were taken for culture and nucleic acid testing by polymerase chain reaction (PCR) for Bordetella pertussis. Immunoglobulin A and immunoglobulin G antibodies against pertussis toxin (PT) were tested by ELISA in paired serum samples. A total of 148 patients were recruited during 1 y. Pertussis was detected in 25 (16.9%) patients with at least 1 of the tests. PCR was positive in 12 patients, and 9 cases was diagnosed serologically. Both PCR and serology were positive in 4 children. Duration of cough was longer in the patients with pertussis (median 33 vs 20, p=0.03). Seropositivity of pertussis toxin was higher in pertussis negative patients during enrollment (24% vs 65%, p=0.005). From the results of this study, B. pertussis seems to be common in our population despite high immunization rates with whole cell vaccine. Although the duration of cough is defined as longer than 21 d in some studies for pertussis case definition criteria, it was shorter than this in 3 of our cases.