Preterm labor and preterm premature rupture of the membranes (PROM) remain important and potentially preventable causes of prematurity. Colonization with Ureaplasma urealyticum has been associated with prematurity and other adverse pregnancy outcomes in several case-control studies. However, prospective studies have not found an association between maternal colonization with U. urealyticum and PROM or prematurity. Differences in study design, failure to identify and adjust for other potential risk factors for prematurity and the frequency of concurrent genital infections with other pathogens make the interpretation of these studies difficult. Recent studies suggest that the increased intravaginal concentration of U. urealyticum and other microorganisms associated with bacterial vaginosis may be associated with PROM, preterm labor and amniotic fluid infection and may be a more important determinant of adverse pregnancy outcome than the simple qualitative recovery of the microorganisms from the maternal genital tract.