Pneumonia caused by Chlamydia psittaci likely occurs considerably more frequently than is suggested by the mere 100 to 200 cases reported annually to public health authorities. Parrots and other psittacine birds still are regarded as the major reservoir of the infectious agent and most recognized cases are associated with owning pet birds or working in a pet store. It was once thought that birds imported from abroad, often illegally, were a principal source, but many domestic breeder flocks of pet birds now have become infected. There is little that is distinctive in the clinical presentation of psittacosis. Therefore, a careful history of bird contact is often the first clue to the diagnosis. The diagnosis frequently is first considered in evaluating a patient whose pneumonia has not responded to therapy with a beta-lactam antibiotic. The diagnosis usually is established serologically. Tetracycline is the preferred therapy and is administered for three weeks to prevent relapses.