Ureaplasma urealyticum is commonly isolated from the amniotic fluid of unselected individuals with intact membranes at the time of cesarean section even prior to onset of labor. The risk of placental infection increases with onset of labor, rupture of membranes and number of vaginal examinations. Qualitative cultures of women with clinical signs of intraamniotic infection and matched controls indicate that U. urealyticum can be found in 50% of amniotic fluid samples of both groups thus suggesting that it may not be a cause of clinical amnionitis. To gain further understanding of the potential role of this organism, we performed amniotic fluid and blood cultures and measured serologic responses by enzyme-linked immunosorbent assay in women with clinical intraamniotic infection and matched controls. U. urealyticum was the single most common bacterial species isolated from maternal blood and amniotic fluid but the isolation rate did not differ between symptomatic and asymptomatic women. Also other known pathogenic bacteria were often isolated from amniotic fluids containing ureaplasmas. However, the marked difference in serologic response between symptomatic and asymptomatic women and the occurrence of ureaplasmemia in some suggest that in certain individuals U. urealyticum may be a cause of clinical amnionitis. Serologic responsiveness, ureaplasmemia and isolation of ureaplasmas in pure culture from amniotic fluid of some asymptomatic women suggest that U. urealyticum may also be a cause of clinically silent amnionitis. Previous studies have shown a significant association between chorioamnionitis documented by histopathology and isolation of U. urealyticum from the placenta or infant but not the maternal cervix.(ABSTRACT TRUNCATED AT 250 WORDS)