Pneumonia, which usually requires hospitalization for children, is caused by various pathogens. According to recent surveys, the prevalence of atypical pneumonia caused by Mycoplasma or Chlamydia has increased, especially in preschool children. Also, the evidence has been accumulated that Mycoplasma or Chlamydia infection is associated with asthma including both inception and exacerbation. Therefore, it is important to consider how the clinical aspects of Mycoplasma pneumonia have changed. In particular, Chlamydia pneumonia, which has not been prevalent in Korea, needs greater attention.