The association of Chlamydia pneumoniae with pneumoniawasfirst reported in 1985. This review summarizes the data collected during the subsequent 10 years on the prevalence, clinical features, diagnosis, and treatment of this disease, whichis nowassociatedwith ∼10% of all casesof pneumonia worldwide. Primary infections are documented most often in schoolchildren and young adults, while reinfections are prominent among the elderly. C. pneumoniae pneumoniais difficult to diagnose; its onset is often insidious, with nonpurulent sputum and without leukocytosis. The microbiologicaldiagnosis is based on the results of serological tests, polymerase chain reaction,and culture. Tetracyclines and macrolides are effective in the treatment of C. pneumoniae pneumonia, as are new quinolones.