Five outbreaks of infection (three pertussis, one parapertussis, and one mixed) in schools were studied prospectively. Nasopharyngeal swabs were obtained from a total of 697 children for culture of Bordetella organisms. Of 50 vaccinated children with culture-confirmed Bordetella infections (29 with pertussis and 21 parapertussis), 40 were symptomatic and 10 remained symptom-free. Smaller numbers of colonies were recovered from the nasopharyngeal swabs of the asymptomatic children than from those of the symptomatic children. Older children had longer durations of illness than younger ones. Our results indicate that during outbreaks children who do not develop disease may have small amounts of Bordetella organisms in their nasopharynges and/or better immune defenses against the disease.