The purpose of this report is to examine the association of histologic chorioamnionitis with microorganisms isolated from the fetal membranes and to evaluate whether microorganisms with or without inflammation are associated with labor characteristics and with pregnancy complications. Inflammation was more common among membranes that yielded pathogenic bacteria (47%, P = 0.002) or Urea-plasma urealyticum (34%, P = 0.03) than among membranes that yielded no growth or nonpathogenic bacteria (20%). Prolonged membrane rupture (P = 0.0001), infant birth weight less than 2500 g (P = 0.02), and intraamniotic infection (P = 0.001) occurred more frequently among those women whose membranes yielded pathogenic bacteria than among those whose membranes yielded no growth or nonpathogenic bacteria. Our findings suggest that placental membranes in which pathogenic bacteria are accompanied by inflammation are associated with the highest risk of pregnancy complications.