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Background: Neutrophilic asthma is thought to be less responsive than eosinophilic asthma to anti-inflammatory therapies including corticosteroids. Chlamydia pneumoniae has been implicated in asthma, possibly by induction of interleukin (IL-8). We hypothesized that IL-8 is increased in the bronchoalveolar lavage (BAL) fluid from children with asthma and C. pneumoniae., Methods: BAL fluid was analyzed for C. pneumoniae and IL-8 using polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay from 2 asthma patient populations in the Bronx, NY and Massachusetts with an average age of 8 and 8.7 years old, respectively. For comparison, samples were also analyzed for C. trachomatis and Mycoplasma 16s DNA., Results: Of 18 Bronx samples analyzed, 6 (33%) were PCR-positive for C. pneumoniae, 10 (56%) for C. trachomatis, and 8 (44%) for Mycoplasma 16s DNA. IL-8 from C. pneumoniae-positive samples was 3.3-fold higher compared with negative samples (P = 0.003). There was no difference between patients tested for C. trachomatis or Mycoplasma. Of 84 Massachusetts samples analyzed, 42 (50%) were PCR-positive for C. pneumoniae, 42 (50%) for C. trachomatis, and 13 (16%) for Mycoplasma. IL-8 concentration from C. pneumoniae-positive samples was 10.49-fold higher compared with negative samples (P = 0.0001). As in the Bronx cohort, there were no differences between patients tested for C. trachomatis or Mycoplasma. Lastly, BAL neutrophilia predicted the presence of C. pneumoniae but not Mycoplasma or C. trachomatis., Conclusions: Children with asthma who were PCR-positive for C. pneumoniae demonstrated elevated concentrations of IL-8 and neutrophils in BAL fluid compared with similar patients who were positive for C. trachomatis or Mycoplasma organisms, but PCR-negative for C. pneumoniae. Undiagnosed C. pneumoniae infection in children may therefore contribute to poorly controlled asthma via induction of IL-8.