A prospective study is presented which addresses the relative effect of cervicovaginal infection and precocious maturation of the uterine cervix on preterm delivery. From April 1981 through December 1983, a total of 5758 pregnant women were checked by means of a vaginal examination at every prenatal visit and a research for bacterial cervicovaginal infection whenever abnormal signs were observed. The study reveals that vaginal infection has no measurable effect when observed during the second trimester of pregnancy, and a small effect during the third trimester. This means that infection of the vagina or/and the cervix may be demonstrated as a risk factor only when the cervix is short before 28 weeks or open before 37 weeks.