Chorioamnionitis is an inflammation of the placental membranes induced by microorganisms which reach the endometrial cavity from the vagina and uterine cervix. Premature labor frequently depends on infections. In patients with premature rupture of membranes (PROM) antibiotic treatment can prevent intra-amniotic inflammation if it is absent at admission. In spite of antibiotic treatment started immediately after the PROM in 218 patients, the chorioamnionitis did not prevent delivery in 41 patients within 48 hours of PROM. In the presence of a previous chorioamnionitis, antibiotic treatment cannot prevent premature labor, whereas it can prevent infection and lead to a longer duration of pregnancy if PROM does not depend on previous infection.