OBJECTIVE: Our purpose was to determine whether amniotic fluid interleukin-6 is increased and inversely proportional to gestational age in women with chorioamnion colonization and spontaneous labor versus women delivered for medical or obstetric indications. STUDY DESIGN: The chorioamnion and amniotic fluid were cultured at cesarean delivery for aerobic and anaerobic bacteria, fungi, mycoplasmas, Chlamydia trachomatis and Trichomonas vaginalis in 269 women with singleton gestations with intact membranes. The amniotic fluid interleukin-6 concentration was also determined. RESULTS: Amniotic fluid interleukin-6 levels were (1) higher in women with spontaneous labor versus those with indicated deliveries (15.8 ± 5.0 vs 2.2 ± 0.2 ng/ml, p = 0.01), (2) inversely proportional to gestational age in women with spontaneous labor (<34 weeks: 47.4 ± 18.0 ng/ml vs ≥34 weeks: 8.7 = 4.1 ng/ml, p = 0.001) but not in women with indicated deliveries (1.5 ± 0.4 vs 2.4 ± 0.3 ng/ml), (3) higher in women with a positive versus a negative chorioamnion (15.1 = 4.8 vs 3.0 ± 0.8 ng/ml, p < 0.001) or amniotic fluid (17.4 ± 7.7 vs 3.8 ± 0.9 mg/ml, p < 0.001) culture, and (4) higher in women with a negative amniotic fluid but positive chorioamnion culture compared with women in whom both cultures were negative (10.0 = 4.4 vs 3.0 ± 0.9 ng/ml, p = 0.002). CONCLUSIONS: Amniotic fluid interleukin-6 levels are (1) higher and inversely proportional to gestational age in women with intact membranes and spontaneous labor versus indicated deliveries, (2) higher in women with one or more microorganisms in the chorioamnion or amniotic fluid, and (3) reflective of chorioamnion microbial colonization, even when the amniotic fluid culture is negative, and may be a useful clinical marker for infection-mediated preterm labor. © 1995, All rights reserved.