['Subclinical intra-amniotic infection is often associated with preterm delivery and may precede it by several weeks. We tested the hypothesis that Interleukin-6 (IL-6) may be elevated in the midtrimester amniotic fluid of pregnancies destined to deliver preterm.', 'A historical cohort study was designed to compare the amniotic fluid (AF) concentrations of IL-6 at 14-20 weeks in a group of women subsequently delivering at < or = 34 weeks (n = 13) with those of women delivering at term (n = 166). Included were singleton gestations with no evidence of fetal structural or chromosomal abnormalities, or maternal conditions known to be associated with preterm delivery (n = 179). Levels of IL-6 were measured by immunoassay and correlated with demographic and pregnancy outcome information. Statistical analysis included correlation, one-way ANOVA after log-transformation, contingency tables, logistic regression, and receiver operator characteristic (ROC) curve analysis.', 'There was an inverse correlation between AF IL-6 levels at 15-20 weeks and gestational age at delivery (r = -0.16, P = 0.03). Women delivering at < or = 34 weeks had significantly higher median AF IL-6 levels (570 pg/ml versus 330 pg/ml, P < 0.0001), rate of African American race (50% versus 12%, P = 0.004), and of infants with birth weights < 10th centile (31% versus 7%, P = 0.02) than women delivering at > or = 37 weeks. Logistic regression analysis showed that IL-6 was independently associated with PTD at < or = 34 weeks after controlling for race and birth weight centiles (P = 0.039).'] Are elevated amniotic fluid interleukin-6 levels during the early second trimester associated with greater risk of subsequent preterm delivery?