Chlamydia pneumoniae and Mycoplasma pneumoniaeare common causes of respiratory infection among teenagers and adults, and may cause a higher proportion of cases of community-acquired pneumonia among children than previously estimated. Further evidence supporting an association between C. pneumoniae infection and the chronic conditions of asthma and atherosclerotic cardiovascular disease have recently been reported. M. pneumoniae has caused several recent outbreaks, the delay in the recognition of which emphasizes the need for a rapid diagnostic test.