Chlamydia pneumoniae is a recently recognized human respiratory pathogen with a unique biphasic life cycle characterized by an obligate intracellular (replicative) and an extracellular (infectious) form of the organism. C. pneumoniae is widely distributed and, via the respiratory route, infects the majority of the world's population. The majority (70%) of acute human C. pneumoniae respiratory tract infections are asymptomatic or only mildly symptomatic but a minority (30%) cause more severe respiratory illnesses including community-acquired pneumonia, bronchitis and a variety of upper airway illnesses. After acute infection the C. pneumoniae intracellular life cycle is characterized by the development of metabolically inert (and thus antibiotic resistant) atypical "persistent" inclusions; this biologic behavior correlates with a clinical course following acute symptomatic illness that is characterized by persistence of symptoms that are difficult to treat with antibiotics. A role for C. pneumoniae in chronic respiratory illness is currently under investigation: "persistent" intracellular inclusions contain increased quantities of chlamydial heat shock protein 60 (hsp 60), a highly immunogenic protein that has been implicated in the pathogenesis of established chronic inflammatory chlamydial diseases (blinding trachoma, pelvic inflammatory disease and tubal infertility). An emerging body of evidence, including host immune response to chlamydial hsp 60, links C. pneumoniae infection with a spectrum of chronic inflammatory lung diseases of currently unknown etiology (asthma, chronic bronchitis and chronic obstructive pulmonary disease (COPD)). Further laboratory developments, including reliable and practical diagnostic methods and antibiotics effective against persistent infection, will be required to recognize and treat acute C. pneumoniae infection, and to advance our knowledge and understanding of the role of chronic infection in asthma, chronic bronchitis and COPD.