Objective: Our purpose was to compare angiogenin, lactate dehydrogenase (LDH) and fibronectin levels in mid-trimester amniotic fluid of patients with preterm and term deliveries and to find out their predictive values for preterm birth. Study Design: A prospective cohort study was conducted in 55 pregnancies with singleton gestations that underwent amniocentesis at 15-20 weeks for standard genetic indications. Amniotic fluid angiogenin, lactate dehydrogenase and fibronectin levels were measured by enzyme-linked immunosorbent assay (ELISA), radial immundiffusion technic and automated analyzer, respectively. Results: Five patients delivered preterm, five developed signs or symptoms of threatened preterm, labor and 45 had term delivery after an uneventful pregnancy. Demographic data were not significantly different. Amniotic fluid angiogenin and lactate dehydrogenase levels were significantly higher in patients with preterm than term deliveries (P < 0.001 and 0.02, respectively). Receiver-operator characteristic curve analysis showed that the amniotic fluid angiogenin had the best screening efficiency in predicting preterm delivery. An angiogenin level of 35 ng/ml was the optimal cut-off value for the prediction of preterm delivery, with a sensitivity of 100% and specificity of 91%. Conclusion: Second-trimester angiogenin is found to be quite effective in the prediction of preterm delivery. Preexisting intrauterine ischemia may be an important risk factor for preterm delivery and already be present in the early mid-trimester. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.