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SUMMARYThe human Ureaplasmaspecies are the most frequently isolated microorganisms from the amniotic fluid and placentae of women who deliver preterm and are also associated with spontaneous abortions or miscarriages, neonatal respiratory diseases, and chorioamnionitis. Despite the fact that these microorganisms have been habitually found within placentae of pregnancies with chorioamnionitis, the role of Ureaplasmaspecies as a causative agent has not been satisfactorily explained. There is also controversy surrounding their role in disease, particularly as not all women infected with Ureaplasmaspp. develop chorioamnionitis. In this review, we provide evidence that Ureaplasmaspp. are associated with diseases of pregnancy and discuss recent findings which demonstrate that Ureaplasmaspp. are associated with chorioamnionitis, regardless of gestational age at the time of delivery. Here, we also discuss the proposed major virulence factors of Ureaplasmaspp., with a focus on the multiple-banded antigen (MBA), which may facilitate modulation/alteration of the host immune response and potentially explain why only subpopulations of infected women experience adverse pregnancy outcomes. The information presented within this review confirms that Ureaplasmaspp. are not simply “innocent bystanders” in disease and highlights that these microorganisms are an often underestimated pathogen of pregnancy.