Atypical organisms (Chlamydia pneumoniae, Mycoplasma pneumoniae) have been recently linked to asthma in various ways: an infection with these organisms may precede asthma onset, exacerbate asthma, or make asthma control more difficult. Their ability to elicit a TH2 response and promote airway inflammation may be the common pathway in the development of an atopic inflammatory response. This article presents a summary of the evidence that infection with Chlamydia pneumoniae or Mycoplasma pneumoniae may play a significance role in asthma.