Objectives: 1) To determine the association between maternal characteristics and the microbial flora of the vagina in pregnancies with high risk of prematurity; and, 2) To determine the association between the microbial flora of the vagina and the maternal and perinatal outcome in pregnancies with high risk of prematurity. DESIGN: Case-control study, between January 1994 and December 1995. Material and methods: Case group comprised 238 singleton-pregnant women who were admitted because of preterm premature contractions, preterm labor, increased vaginal fluid, acute pylonephritis and arterial hypertension. All cases underwent a wet preparation to establish the microbial flora. The control group included 3 850 singleton pregnant women who delivered a term neonate at the same institution. Case group was further divided into women with bacterial vaginitis (n=136), bacterial vaginosis (BV, n=...