To evaluate the clinical features of pertussis infection in adults, we examined 33 subjects (11 males and 22 females; mean age, 42.0±16.5 years) who were clinically diagnosed to have pertussis infections according to guidelines of the Japanese Respiratory Society. All subjects were examined for medical history, family history, and respiratory function, and serum samples and nasopharyngeal swabs were collected. The nasopharyngeal samples were analyzed by loop-mediated isothermal amplification (LAMP). The serum specimens were assayed for antibodies to pertussis toxin (PT) by an enzyme-linked immunosorbent assay. Fourteen patients were diagnosed as having a pertussis infection based on the values of the PT and/or the nasopharyngeal swab (LAMP). The other 19 subjects were not diagnosed. The duration of coughing in pertussis subjects was significantly longer than in the other cough subjects. No pertussis subjects had a high fever (>37.5°C). Five subjects (35.7%) developed cough variant asthma or bronchial asthma postpertussis. In adult patients with persistent coughs, especially in those without high fevers, a pertussis infection should be considered as a differential diagnosis.