Clinical and serological studies of chlamydial pneumonia were done in six patients (three men and three women). The other three patients had no avian contact and showed almost the same clinical symptom. Acute infection with Chlamydia psittaci and Chlamydia pneumoniae were diagnosed in two patients and in one patient, respectively, by MFA. Because in some cases Chlamydia psittaci pneumonia and Chlamydia pneumoniae pneumonia are difficult to differentiate, it is necessary to use a test that allows different chlamydia species to be distinguished.