The aim of the following work was to evaluate vaginal and cervical colonization in patients with threatening preterm labor and comparing the frequency of colonization of the term and preterm labor groups.532 pregnant women with threatening preterm labor were included into the clinical trial. The frequency of colonization was established and the frequency of colonization depending on the duration of pregnancy was checked.Positive vaginal cultures were found in 29.5% of patients. The most frequent were: Escherichia coli and Streptoccocus agalactiae. There was no relationship between the duration of pregnancy the frequency of colonization or the type of bacterial culture.30% of positive vaginal and cervical cultures may indicate about the deficiency of the used method. The most frequent microorganisms remain to be Candida, Escherichia coli and GBS. However, the frequency of colonization with rare bacteria increases. Not only bacterial colonization but a group of different factors may be the reason of preterm labor.