To describe the clinical course of Bordetella pertussis infection in a highly immunized childhood population, we studied prospectively endemic and epidemic pertussis in a metropolitan population with an immunization rate > 90% during an 8-year period from 1987 through 1994. Patients with a possible diagnosis of pertussis were referred by family or emergency room physicians for nasopharyngeal culture. Patients with a culture positive for B. pertussis were contacted by a nurse who completed a detailed questionnaire for the index case and all family members. Repeat home visits were made each week for 4 weeks. Of the 189 patients with pertussis who were evaluated 103 subjects were < 5 years of age. Congestion predated the onset of cough by up to 1 week in 35 (34%) cases. Seventy (68%) subjects < 5 years of age developed a paroxysmal cough within the first week of their illness. Ninety-one (88%) cases < 5 years old had a persistent paroxysmal cough for > 21 days. Coughing in this group lasted from 16 to 91 days (median 48). Erythromycin therapy appeared to shorten the duration of cough; however, patients were not randomized to receive erythromycin at a specific time. Despite adequate immunization some children develop pertussis. The clinical course in these patients is milder than in unimmunized subjects. Nevertheless the symptomatology in these children should still be readily identified by most physicians using classical clinical criteria of pertussis.