PURPOSE: to study cervical colonization in women with preterm labor or premature rupture of membranes. METHODS: two hundred and twelve pregnant women with preterm labor or premature rupture of membranes were studied. Two cervical samples from each woman were collected and bacterioscopy and culture were performed. Association of cervical microorganisms and urinary tract infection, chorioamnionitis, fetal stress, antibiotic use, prematurity, neonatal infection, and neonatal death were evaluated. RESULTS: the prevalence of endocervical colonization was 14.2% (CI95%=9.5-18.9%), with similar results in preterm labor or premature rupture of membranes. Group B streptococcus was the most prevalent organism (9.4%). Other organisms isolated were Candida sp, Streptococcus sp, Streptococcus pneumoniae, Escherichia coli and Enterococcus sp. The most common findings of bacterioscopy were a reduced number of lactobacilli and a great number of leukocytes. Endocervical colonization was associated with a higher occurrence of urinary tract infection (23.8 versus 5.4%; p<0.01), early-onset neonatal infection (25.0 versus 7.3%; p<0.01) and neonatal mortality (two cases in colonized women; p<0.02) when compared with a negative culture of endocervical mucus. CONCLUSIONS: this study showed high prevalence of endocervical colonization despite the use of a nonselective culture media. The main microorganism isolated was group B streptococcus, but other organisms were present in one third of the studied population. 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