Abstract:Abstract To investigate the Chlamydophila pneumoniae infected rate. Blood samples were collected from 173 respiratory tract infection patients and 52 healthy volunteers. DNA was extracted from PBMC and sera were isolated from the blood samples. The MOMP gene of C. pneumoniae were detected by PCR from the DNA samples and C. pneumoniae specific IgG were measured by ELISA. PCR products were detected by 1.5% agarose gel electrophoresis and sequenced by Sangon Company. Combined with the clinic data, the difference of C. pneumoniae infected rate in different age group were analysised. The results of PBMC extrct as follow: 173 people of the respiratory tract infection group, the PCR(MOMP) positive rate is32.40%(56/173). 52 people of the healthy group, the PCR(MOMP) positive rate is 9.62%(5/52).The MOMP PCR products with Cpn AR-39 of the MOMP gene 99% consistent. According to the positive rate of each group, it can be seen that C. pneumoniae infected rate of aged group is the highest. Cpn is one of the most important respiratory tract pathogen proved to be the main pathogen for C. pneumoniae. Old people are of high-risk group for C. pneumoniae infection.