A number of organisms, including Mycoplasma, group B Streptococcus, Bacteroides, Neisseria gonorrhoeae and Chlamydia trachomatis, have been isolated more frequently from patients in premature labor than from controls. Prophylactic antibiotic treatment in some studies lowered the incidence of prematurity. Silent chorioamnionitis has been noted in 15% of patients in premature labor. Untreated pyelonephritis is clearly associated with premature labor; however, the association of asymptomatic bacteriuria, appropriately treated pyelonephritis and premature labor is less clear. Some microorganisms have been demonstrated to produce phospholipase A2 and possibly prostaglandins, which might be the mechanism for some of the associations between premature labor and bacteria.