Preterm birth still remains a significant management problem, and a large number of markers of the disease have been investigated. We measured plasma levels of urocortin, a neuropeptide expressed by gestational tissues, in women with threatened preterm labor (TPTL) to evaluate whether the measurement may predict preterm delivery (PTD). We studied patients as part of an open observational study. The study was conducted at a tertiary referral center for obstetric care. Eighty-five women with singleton pregnancies between 28 and 34 completed gestational weeks with TPTL participated in the study. Interventions included clinical examination and urocortin measurement. Pregnancy outcome and evaluation of sensitivity, specificity, and predictive values of urocortin as diagnostic test for PTD were measured. Thirty of 85 patients (35.3%) had PTD: 23 of 30 delivered within 7 d from admission (delivery time interval: 2.91 +/- 1.62 d; gestational weeks at delivery: 32.12 +/- 1.7); the remaining delivered later (delivery time interval: 11.71 +/- 4.27 d; gestational weeks at delivery: 33.5 +/- 2.18). Urocortin was significantly higher in women who delivered preterm (median 131.2 pg/ml, interquartile interval 115.1-139.4 pg/ml) than in those who progressed to term delivery [95.4 (69.9-101.3) pg/ml, P < 0.0001] and still higher in those delivering within 7 d from admission [137.7 (124.8-141.2) pg/ml]. Receiver operating characteristic curve analysis revealed that urocortin at the cutoff of 113.9 pg/ml had sensitivity of 80%, specificity of 100%, positive predictive value of 100%, and negative predictive value of 90% as a marker for PTD. Maternal plasma urocortin concentration is increased in patients with TPTL who have PTD, and its measurement may be a promising new biochemical marker of PTD.