Evidence related to the association of Ureaplasma urealyticum with premature birth is reviewed. U. urealyticum is a common inhabitant in the vagina of pregnant women. It is recovered from the vaginas of approximately 70% of pregnant women. The presence of U. urealyticum in the vagina is not associated with premature birth. Therapy to eradicate U. urealyticum from the vagina during pregnancy does not reduce the incidence of premature birth. Recovery of U. urealyticum from the amniotic fluid has not been associated with prematurity. However, in three of six reports, the recovery of U. urealyticum from the chorioamnion has been associated with prematurity, and U. urealyticum is rather consistently associated with histological chorioamnionitis. This review suggests that U. urealyticum in the lower genital tract is not associated with premature birth but that its presence in the chorioamnion is weakly associated with prematurity and strongly associated with histological chorioamnionitis.