Abstract Background: The prevalence of Chlamydia trachomatis infections among Polish teenagers remains unknown. Sexually active teenagers are exposed at sexually transmitted infections, including Chlamydia trachomatis (CT). Most infections are asymptomatic and, therefore, untreated. Early detection and treatment of cervical chlamydial infections may prevent pelvic inflammatory diseases and prevent later infertility. Aim:Tto determine the prevalance and risk factors of Chlamydia trachomatis infections among sexually active female teenager. Materials and methods: 243 sexually active girls, 16-19 year-olds, attending the outpatient clinic were enrolled in this study between 2005-2007. The participants filled in a questionnaire containing such information as age, purpose of visit, level of education, age at the first intercourse, number of sexual partners, number of current partners (in the past 3 months), contraceptive methods, use of condoms, past history of STD and obstetric history at the first visit. Pelvic examination was performed to check the following: vaginal discharge, presence of abnormalites of the cervix (ectopy, erythema, tenderness of uterine and adnexal) and to take a Pap smear and a cervical swab for Chlamydia trachomatis. Cervical swabs for Chlamydia trachomatis were tested by polymerase chain reaction (PCR). Results: The prevalence of CT genital infection in the studied group was 2,9%. Adolescent females infected by CT less frequently admitted to the use of condom and more often did not use any contraception at all, in comparison with the girls without CT genital infections (29% and 57% vs. 37% and 19%). There were statistically significant differences in the results of the abnormal Pap smears (ASCUS, LGSIL) between the two groups.