To evaluate the relationships between gestational age, neonatal outcome, and amniotic fluid (AF) bacteria, we obtained AF from women with intact membranes in idiopathic preterm labor. Positive cultures were obtained from 20 (19%) of 105 women. The frequency of positive cultures was inversely related to gestational age: 23-26 weeks, nine of 20; 27-30 weeks, four of 24; and 31-34 weeks, seven of 61 (chi2 for trend, P less than .001). Fusobacterium nucleatum, Bacteroides ureolyticus, and Ureaplasma urealyticum were the most common isolates. Facultative and anaerobic bacteria were more commonly isolated from women at less than 30 weeks' gestation, and Ureaplasma urealyticum was commonly isolated at greater than 30 weeks' gestation. Forty percent of the patients identified as having positive AF facultative and anaerobic cultures by the research laboratory had negative cultures in the clinical laboratory. Clinical characteristics and maternal white blood cell count and differential did not differ between women with and without positive cultures. Elevated C-reactive protein levels and a positive AF Gram stain were the two most sensitive and specific methods to predict positive AF cultures. Women with positive cultures delivered a median of 1.0 day after enrollment, compared with 28.5 days for women with negative cultures. The median gestational age at delivery for women with positive cultures was 27.5 weeks, and the median birth weight was 866 g. Positive AF cultures were associated with respiratory distress syndrome, bronchopulmonary dysplasia, and neonatal death. If occult AF infection among women in preterm labor is a treatable cause of preterm birth, then treatment could markedly reduce both perinatal morbidity and mortality.