Objective: To determine the clinical significance of meconium-stained amniotic fluid (AF) observed at amniocentesis in patients with preterm labor. Methods: A nested case-control study was constructed based on the color of AF during amniocentesis. Forty-five women admitted with preterm labor and meconium-stained AF were matched for gestational age at admission and compared with 135 women with preterm labor and clear AF. All AF samples were cultured for aerobic and anaerobic bacteria and mycoplasma. Results: The rates of positive AF cultures for microorganisms, overall preterm birth (before 36 weeks), preterm birth before 32 weeks, and clinical chorioamnionitis were all significantly higher in patients with meconium-stained AF than in those with clear AF (positive AF cultures, 38 versus 11%, P < .001; preterm delivery before 36 weeks, 73 versus 41%, P < .001; preterm delivery before 32 weeks, 51 versus 17%, P < .001; and clinical chorioamnionitis, 22 versus 6%, P = .003). In contrast, no significant differences were observed between groups with respect to maternal age, gravidity, parity, abruptio placentae, placenta previa, fetal distress, cesarean rate, or puerperal morbidity. Conclusion: Patients with preterm labor and meconiumstained AF had higher rates of microbial invasion of the amniotic cavity, clinical chorioamnionitis, and premature deliveries than those with clear AF.