We investigated Ureaplasma urealyticum genital tract colonization rates in an Australian population to determine whether colonization was associated with adverse pregnancy outcome. Women attending an antenatal clinic were evaluated for lower genital tract colonization at their first antenatal visit (162 women) and at 28 weeks' gestation (120 women). Placentas from 92 women were cultured. U. urealyticum was the predominant isolate from the lower (57.4%) and upper (17.4%) genital tract in this population of pregnant women. U. urealyticum was a persistent colonizer during mid-trimester of pregnancy (in 88% of women colonized) whereas M. hominis, G. vaginalis, and Group B streptococcus were present as transient flora of the lower genital tract. Lower genital tract colonization during pregnancy was not directly associated with adverse pregnancy outcome. However preterm delivery in afebrile, asymptomatic women, could possibly be associated with chorioamnionitis (4 of 16 preterm births). Screening of women with a history of preterm birth may prevent upper genital tract infections and preterm delivery.