Chlamydia pneumoniae, a fairly recent addition to the genus Chlamydia, has been shown to cause community-acquired and nosocomial pneumonias, and to be a common infectious agent in human populations throughout the world. It has a wide variety of clinical presentations, and may be associated with ischaemic heart disease. Like other species in the genus, it is difficult to isolate by cell culture: clinical diagnosis therefore rests on other techniques, each of which presents particular problems. Those discussed in this review are cell culture, antigen detection by direct immunofluorescence and enzyme immunoassay, serological methods (exemplified by micro-immunofluorescence) and the polymerase chain reaction. Currently available treatments and the prospect of vaccination are also discussed.