Background. To evaluate the accuracy of the decidual phosphorylated isoform of insulin‐like growth factor binding protein‐1 in endocervical secretions to predict premature delivery in symptomatic and asymptomatic pregnant women. Methods. The study included 332 pregnant women: 109 symptomatic patients (study group) and 223 asymptomatic women (control group). For all women, qualitative and quantitative assessment of the decidual phosphorylated isoform of insulin‐like growth factor binding protein‐1 in endocervical secretions was carried out, but the quantitative assay was finally performed in 282 of 332. Student's test, the φ2 test, and Fisher's exact test were used as appropriate. Results. The phosphorylated insulin‐like growth factor binding protein‐1 qualitative test was positive in 35 of 301 women (11.6%) and 11 (31.4%) of them delivered before 37 weeks; in the remaining 266 women with a negative test, there were 11 (4.7%) premature deliveries (relative risk=5.8; 95% CI=3.3–10.3). The mean values of quantitative phosphorylated isoform of insulin‐like growth factor binding protein‐1 were 56.9 µg/l (95% CI=40.7–73.1) in cases of a positive qualitative test and 6.1 µg/l (95% CI=4.0–8.3; p = 0.0001) in women with a negative result. The sensitivity, specificity, positive predictive value, and negative predictive value for phosphorylated isoform of insulin‐like growth factor binding protein‐1 test in symptomatic patients were 69.2%, 90.5%, 50%, and 95.6% respectively, while in the asymptomatic patients they were 22.2%, 91.8%, 11.8%, and 96% respectively. Conclusions. The phosphorylated isoform of insulin‐like growth factor binding protein‐1 in cervical secretions is a potential specific marker for preterm delivery occurring before 37 weeks. This test may have an important role in the management of women presenting with symptoms suggestive of preterm labour.