['In the United States (U.S.), Bordetella pertussis incidence has increased. Cough and apnea are common findings in pertussis and also in bronchiolitis, the most common cause of hospitalization in U.S. infants. The objective was to determine the prevalence of B. pertussis infection in children hospitalized with bronchiolitis and to describe its clinical course.', 'Children hospitalized with bronchiolitis and age <2 years were eligible for a prospective, multicenter cohort study during 3 consecutive winter seasons (November-March) from 2007 to 2010. Sixteen sites in 12 states participated using a standardized enrollment protocol. Families were asked the 2010 Centers for Disease Control and Prevention pertussis classification questions. Nasopharyngeal aspirates were obtained and tested by real-time polymerase chain reaction for 16 viruses, Mycoplasma pneumoniae and B. pertussis.', 'Two thousand sixty-eight (94%) of 2207 children had 1 or more respiratory pathogens. B. pertussis was identified in 4 children [0.2%; 95% confidence interval (CI): 0.1-0.5%] with 3 having a viral co-infection. All 4 were younger than 4 months; 2 met the Centers for Disease Control and Prevention definition of probable pertussis; and 3 had received at least 1 dose of an acellular pertussis vaccine. During the hospitalization, 2 had paroxysmal cough, 1 required intensive care unit care and the median length of stay was 13 days.'] Is bordetella pertussis an uncommon pathogen in children hospitalized with bronchiolitis during the winter season?