Objective. To examine the reliability of human placental alkaline phosphatase (hPlAP) in the mother's serum as a marker of premature labour among pregnant women who had no known risks for premature labour and to determine the critical value of hPlAP in pregnancies which ended up as a premature labour. Patients and Methods. The research was conducted in the form of a prospective study of 200 pregnant women. All the pregnant women were divided into two groups, the examinees and the control group. The value of hPlAP in serum of all pregnant women determined in the period from week 20 to 24 of gestation. > 2 median value was taken as a critical value for hPlAP . Besides descriptive statistical methods for the statistical data processing we used the χ2 test, student t-test, Fishers test and Mann-Withneys test, logistic regression. Results. The number of premature labours in the examined group was 17 (11.3%), in the control group 22 (44%). The probability of premature labour is 6.1 times higher in the control group in relation to the examined group. The mean value of hPlAP in the examined group was 608.2 but in the control group 1115.6. The mean value of hPlAP in the pregnant women who gave birth prematurely was 1195 but in those who gave birth on time 632.2. There was a statistical significant difference in mean values of hPlAP. Conclusios. hPlAP can be used as a reliable marker of idiopathic premature labour. hPlAP values connected with the development of premature labour is 990 mU/l. [ABSTRACT FROM AUTHOR]