In order to know the incidence of Chlamydia pneumoniae (strain TWAR) pneumonia and its clinical features, 93 patients with pneumonia and 93 matched patients with non-respiratory diseases were studied. TWAR antibodies (IgG and IgM) were detected by microimmunofluorescence (MIF) test. The results showed that 19.4% (18 cases) patients with pneumonia were TWAR pneumonia, in which 10 cases accompanied by bacteria infection and 7 cases being simple TWAR pneumonia. There were no significant differences in clinical features between TWAR pneumonia and non-TWAR pneumonia, except dry and moist rales. These data showed that the occurrence percentage of TWAR pneumonia in patients with lung cancer was higher than that in patients with the other respiratory diseases. This study suggests that there are TWAR pneumonia in China.