Chlamydia pneumoniae, a Gram-negative bacterium, formerly named TWAR but identified as a distinct species since 1988, is now considered to be the most common agent of chlamydial infection in Scandinavia. C pneumoniae has a different tissue trophism from that of Chlamydia trachomatis, since C pneumoniae may infect bronchi and lungs, macrophages, monocytes, and endothelial cells. C pneumoniae, like other chlamydiae, has a slow, intracellular life cycle. An absence of reaction from the host cells, combined with scant tissual reaction owing to the low endotoxic activity of chlamydial lipopolysaccharide, may help to explain the usually discreet clinical picture. Atherosclerosis and coronary heart disease may follow chronic lung infection, and acute pneumonic episodes can trigger myocardial infarct. Asymptomatic infection with C pneumoniae is widespread. Intriguing diagnostic questions are the possible existence of a non-pathogenic carrier state, and the conceivable sensitization of the host with respect to a heterotypic, secondary chlamydial infection by, for example, C trachomatis, giving rise to an aggravated clinical picture. Early antibiotics are indicated to avoid the development of chronic disease.