Chlamydia are obligate intracellular bacteria with a biphasic life cycle. In 1986 a new species of Chlamydiae, Chlamydia psittaci TWAR was discovered, later named Chlamydia pneumoniae. C. pneumoniae is probably an important pathogen causing respiratory tract infections in man, ranging from subclinical infection to life threatening pneumonia. Infections can occur in epidemics. Furthermore, associations between C. pneumoniae infection and coronary artery disease, lethal myocarditis and sarcoidosis have been suggested. The current diagnostic method is based on detection of specific antibodies against C. pneumoniae in blood. Macrolides or tetracyclines are the drugs of choice for treatment of C. pneumoniae infection.