Since the description in 1986 of Chlamydia pneumoniae as a pathogen it has become recognized as a common infectious agent all over the world. C. pneumoniae is primarily a respiratory tract pathogen that causes approximately 10-20% of community acquired pneumonia in adults and children, and 10-20% of acute bronchitis in adults. It also causes sinusitis, primary pharyngitis, and may trigger for asthma. Most of infections with this micro-organism are in fact subclinical and asymptomatic and only rarely cause on overt disease. Chronic infection with C. pneumoniae has been suggested as a factor in the development of atherosclerosis. Seroepidemiologic studies in different populations suggest that the seroprevalence increases sharply in young children and adolescence. After adolescence the seroprevalence continues to increase and may achieve almost complete saturation for IgG and IgA-class antibodies in the senescence. To date, most investigations have relied on serologic diagnosis. Early studies have been performed with a complement fixation (CF) test which is genus-specific and is more likely to be positive in initial infection than during reinfection. The microimmunofluorescence (MIF) method is species-specific, but requires a skilled interpreter and is not suitable for automation and high volume testing. Those technical problems are avoided with the EIA methods developed by Ani Labystems, providing easy, fast and objective tests.