BACKGROUND: Chlamydia trachomatis is most common sexually transmitted pathogen in the world, and a common cause of urethritis and cervicitis. Also it is common cause of preterm premature rupture of membranes and premature labor in pregnant women, and pneumonitis and conjunctivitis in neonate. A rapid and sensitive polymerase chain reaction(PCR)-based assay for detection of C. trachomatis is recently introduced. OBJECTIVES: We studied to determine whether a PCR assay is useful to detect Chlamydial infection in pregnant women. We also studied to compare its prevalence rate according to maternal age, trimester and parity, respectively. Study Design: Specimens were collected from 149 pregnant women by transcervical swab or endocervical lavage. If a specific band was detected in PCR assay, we considered as Chlamydial infection. RESULTS: In general, the positive bands were detected in the 45 of 149 pregnant women(30.2%). The positive bands were detected the 6 of 55(10.9%), 16 of 49(32.7%), and 23 of 45(51.1%) pregnant women in each trimester, respectively. Therefore, there was significantly increased according to the gestational age(p<0.05). However, there were not significantly different according to maternal age and parity(p> 0.05). CONCLUSION: We concluded that the PCR assay is a fast and useful test for the detection of C. trachomatis in transcevical cells from the pregnant women. This study suggested that Chlamydial infection seems to be increased according to the gestational age.