Chlamydia pneumoniae, an obligate intracellular human pathogen, causes infections of the respiratory tract. It is a significant cause of both lower and upper acute respiratory illnesses, including pneumonia, bronchitis, pharyngitis and sinusitis. Most respiratory infections caused by C. pneumoniae are mild or asymptomatic. Some studies have suggested a possible association of C. pneumoniae infection and acute exacerbations of asthma and chronic obstructive pulmonary disease (COPD). Seroepidemiological studies showing antibody prevalence rates in a range of 50 to 70% suggest that C. pneumoniae is widely distributed and that nearly everybody is infected with the agent at some time. C. pneumoniae can cause prolonged or chronic infections which may be due to persistence for months or years. These persistent infections have been implicated in the development of a number of chronic diseases including atherosclerosis, asthma and COPD. These persistent chlamydial infections can be established in vitro using several methods including cytokines, antibiotics and deprivation of certain nutrients. Despite differences in treatment, chlamydiae respond to form inclusions containing atypical reticulate bodies (RBs), which occasionally have been shown to be pleomorphic forms, termed aberrant form (AF). The AF is generally larger in diameter than typical RBs, and display a sparse densinometric appearance. In general, it is likely that this aberrant developmental step leads to the persistence of viable but nonculturable chlamydiae within infected cells over long periods. Removal of several stress factors described above results in the condensation of nuclei, the appearance of late proteins, and the production of viable, infectious elementary bodies (EBs). Most of the major sequelae of chlamydial disease are thought to arise from either repeated or persistent chlamydial infection of an individual. The persistence would allow constant presentation to the individual immune response of these potentially deleterious immune targets. Since repeated infection can certainly be documented in many clinical settings, persistence is thought to also play a role.