From November 1981 to August 1984, 301 adult patients with community-acquired pneumonia were admitted to the major referral hospital of Nova Scotia. Serologic tests done on these patients included microimmunofluorescence using the TWAR strain of Chlamydia and all Chlamydia trachomatis serovars as antigens. The TWAR strain has been shown to cause mild pneumonia in teenagers and young adults. Of the 301 patients, 18 (6%) had serologic evidence of recent infection with the TWAR organism. Their mean age was 64 years. Pneumonia associated with the presence of acute TWAR antibody had no characteristic clinical or radiographic features when compared with pneumonia without acute chlamydia antibody. Six patients, who all had preexisting serious chronic disease, had severe illness, and 2 died. Both patients who died had complicated hospital courses and other concomitant infections. We conclude that the TWAR organism may cause pneumonia in older adults and persons with chronic diseases that require hospitalization.