BACKGROUND: Preliminary evidence suggests that viral-pertussis coinfections are common in nonvaccinated infants. SUBJECTS AND METHODS: Bordetella pertussis infection was studied by polymerase chain reaction in nasopharyngeal aspirates in 142 infants <6 months of age, who were admitted for bronchiolitis. Viral etiology,documented by antigen detection or polymerase chain reaction in nasopharyngeal aspirate, was respiratory syncytial virus (RSV) in 105, rhinovirus in 8, influenza A virus in 8, and other viruses in 10 cases. Only 11 samples were negative. RESULTS: B. pertussis infection was found in 12 (8.5%) cases, being coinfection with RSV in 8 (67%) cases (7.6% of all RSV infections). In a retrospective analysis, RSV-pertussis coinfections and sole RSV infections did not differ for the presence of cough. Preliminary evidence was found that a history of coughing spells was associated with B. pertussis identification. CONCLUSIONS: Coinfection with B. pertussis was present in 8.5% of <6-month-old infants, who were hospitalized for viral bronchiolitis. To avoid underdiagnosis, pertussis should be considered in all nonvaccinated infants admitted for lower respiratory tract infection.