C. pneumoniae has been shown to be a cause of “walking pneumonia” in adults world- wide. It is estimated that 10% of pneumonia and 5% of bronchitis cases are due to this agent. Epidemiologic evidence indicates that infection occurs throughout the year and is spread between humans by person to person contact. Unlike psittacosis, birds are not the reservoir. Outbreaks of community-acquired pneumonia caused by C. pneumoniae have been reported, as has apparent nosocomial spread. Reinfections occur, and clinically evident C. pneumoniae infection may be more evident in the elderly than in younger individuals.Most infections present as pharyngitis, lower respiratory tract disease, or both, and the clinical spectrum is similar to that of Mycoplasma pneumoniae infection. Pharyngitis or laryngitis may occur 1 to 3 weeks prior to bronchitis or pneumonia, and cough may persist for weeks. The diagnosis is established by serologic testing or culture, but these tests are not routinely available. Treatment with tetracycline or erythromycin is effective in amelio-rating the signs and symptoms of C. pneumoniae infection. Currently, there is ongoing scientific interest in the potential role of persistent infection by C. pneumoniae in the pathogenesis of human vascular endothelial and intimal diseases, such as atherosclerosis.