Asthma aetiology is complex, involving interactions between genetic susceptibility, allergen exposure and external aggravating factors such as air pollution, smoking and respiratory tract infections. Available evidence supports a role for acute Chlaymdia pneumoniae or Mycoplasma pneumoniae respiratory tract infection as a trigger for 5 to 30% of wheezing episodes and asthma exacerbations. It also appears that acute infections with C. pneumoniae and M. pneumoniae can initiate asthma in some previously asymptomatic patients; however, the quantitative role for these atypical bacteria as asthma initiators is unknown at the present time. Whether chronic infections with these agents play an important role in persistent asthma symptoms and/or to asthma severity is unclear and additional information should be acquired before definite conclusions can be reached. Improvement in asthma symptoms after antimicrobial therapy active against C. pneumoniae and M. pneumoniae has been observed. In some studies C. pneumoniae seems to be more important for asthma pathogenesis and exacerbations than M. pneumoniae; in other reports the role of M. pneumoniae appears to be more significant. However, a number of questions remain unanswered. Carefully controlled randomised trials are clearly warranted to determine whether infection with atypical bacteria is really associated with asthma and to define the appropriate role of antimicrobial treatment.