Numerous case reports and retrospective studies suggest an association between M. pneumoniae respiratory infection and extrapulmonary complications, the most common of which involve the central nervous system. There is insufficient evidence based on prospective, carefully controlled observations to confirm this association at the present time. A variety of mechanisms has been suggested to explain the involvement of distant organ systems. These include metastatic infection, autoimmunity, toxin generation, and altered host immunity. While none of these is based on evidence to prove an association, the state of anergy which accompanies M. pneumoniae pneumonia deserves consideration and further study as the most plausible link between infecting organisms and extrapulmonary manifestations.