The purpose of this study was to evaluate procalcitonin (PCT) plasma levels in pregnancy complicated by preterm labor and to determine their value in the prediction of preterm delivery and the length of the admission-to-delivery interval. The study population consisted of 53 patients with preterm labor and 31 healthy pregnants. The study patients were divided according to the delivery time and to the admission-to-delivery interval. Plasma PCT concentrations were higher in preterm labor than in healthy pregnants. Although at the onset of preterm labor plasma PCT concentrations in patients who delivered prematurely were higher than in patients who, after tocolytic treatment, delivered at term, the difference was not significant. Also in cases of preterm labor delivered within and after three and seven days of admission no differences were observed. The highest values in the prediction of preterm delivery and the length of admission-to-delivery interval corresponded to a PCT concentration of 1.7 ng/ml. These findings suggest that although preterm labor is associated with increased PCT concentrations in maternal plasma, there is no significant association either between plasma concentration of PCT at the moment of threat and preterm delivery, or the admission-to-delivery interval. The predictive value of plasma PCT determinations is unsatisfactory.