We report the case of a woman who had pneumonia due to Chlamydia psittaci. A Chlamydia species was determined to be the causative agent of the pneumonia because it was isolated from bronchoalveolar lavage fluid, because it could be detected in lung biopsy specimens by the direct immunofluorescence technique, and because Chlamydia-specific antibodies could be detected by ELISA and microimmunofluorescence. The infectious agent could not be identified at the species level with use of serological techniques, but the isolate was determined to be C. psittaci by PCR with use of species- and genus-specific sequences within the chlamydial lipopolysaccharide biosynthesis gene gseA. The case reported herein exemplifies the problems encountered in diagnosing ornithosis and shows that isolation of the etiologic agent followed by identification of the species by PCR is helpful in diagnosing this rare disease. In addition, the findings in our case show that laboratory personnel who are conducting tests for Chlamydia pneumoniae should be aware of the risk of accidentally isolating highly infectious C. psittaci organisms.