Objectives: The study was performed to evaluate the frequency of cervical infections with Chlamydia trachomatis, Ureaplasma urealyticum and Mycoplasma hominis in women with normal pregnancy and with symptoms of spontaneous abortion or preterm delivery. Antioxidant status of pregnant women was assessed. Elevated oxidative status is claimed to be one of the factors causing threatened miscarriage and premature labor. Material and methods: From 73 women endocervical culture was investigated for Chlamydia trachomatis (method of rRNA Hybridization), Ureaplasma urealyticum and Mycoplasma hominis (Mycoplasma IST 2 test). Gestational age ranged from 14-30 weeks. They were categorized into 2 groups: patients not presenting these symptoms without history of pregnancy complications and patients with threatened abortion or symptoms of preterm delivery. Following oxidative status factors were measures: Superoxide Dysmutase, Catalase and Glutathion Peroxidase in blood and saliva of 49 pregnant women of both groups were elevated. Statistica: the Student’s T test, chi-square test, U Manna-Whithney’a test. Results: The results show difficulty in assessment of direct influence of Chlamydia infection on oxidative stress. Ch.trachomatis was present in 9.6%, M. hominis in 6.8%, U. urealyticum in 28.8% of vaginal cultures. Chlamydiasis statistically increases production of Reactive Oxygen Forms what results in unfavorable outcome of the pregnancy. Increase of the following parameters was observed: Superoxide Dysmutase (5583.2 vs 8980.2U/ml/sec) and Catalase (15.1 vs 18.3U/ml/sec). Conclusion: Infection of Chlamydia trachomatis could be a significant factor in pathogenesis of threatened abortion and preterm delivery as it increases oxidative stress.