The chlamydia have long been recognized as ubiquitous pathogens capable of producing a wide spectrum of disease both in children and in adults. Common infections caused by this pathogen include pneumonia in infants, trachoma in children, and genital tract disease in adults. While neonatal chlamydial pneumonia is commonly described, chlamydial pneumonia in adults is less commonly recognized and was originally associated only with exposure to psittacine birds. Instances of person-to-person transmission of a chlamydial pneumonia were first identified in San Francisco in 1941. The organism isolated was a chlamydia-like agent with glycogen-staining characteristics unlike those of the well-studied avian chlamydia but much more difficult to maintain in culture. This finding lapsed into obscurity until 1965, when a chlamydial organism again demonstrating a Chlamydia psittaci-like staining pattern was isolated from a Taiwanese child in a trachoma study. This isolate was eventually named TW183. It was not until 1983, however, when a patient with atypical pneumonia was found to be infected with a similar pathogen, dubbed AR39, that the question of prevalence was raised. As the behavior of these isolates was characterized in Grayston's laboratory, it became apparent that they represented a novel human pathogen and the name TWAR (Taiwan acute respiratory agent) was adopted. Subsequent epidemiologic studies of this agent have shown broad exposure over a wide geographic distribution with almost 40% of tested populations showing serologic evidence of exposure.