Chlamydia pneumoniae strain TWAR, the new third species of Chlamydia, is a common cause of pneumonia and other acute respiratory tract infections. About 10% of hospitalized and outpatient pneumonia cases have been associated with TWAR infection. TWAR is among the four or five most commonly identified causes of all pneumonia. Most TWAR infections are mild or asymptomatic, but occasionally severe pneumonia with death has been observed. Laboratory diagnosis is not generally available. Vigorous treatment with tetracycline or erythromycin is recommended. Both epidemic and endemic infections have been described in North America and the Nordic Countries. Population prevalence antibody studies suggest that TWAR infection is wide-spread throughout the world, that nearly everyone is infected and reinfected during their life-time, and that infection is common in all ages except those less than 5 years in temperate zone countries. The infection is transmitted from person to person, apparently with a long incubation period.