The aim of this prospective study was to compare serum and urine concentrations of progesterone-induced blocking factor (PIBF) and serum concentrations of anti-inflammatory (IL10) and pro-inflammatory (IL6, TNFalpha, IFNgamma) cytokines of women with threatened spontaneous abortion with normal pregnancy and to evaluate the impact of PIBF on outcome of pregnancy. A sample of 30 women with threatened spontaneous abortion (study group) and 20 healthy pregnant women (control group) between 6(th) and 24(th) gestational weeks was studied. Serum and urine PIBF, IL10 and IL6, TNFalpha, IFNgamma cytokine concentrations were measured by enzyme-linked immunosorbent assay (ELISA). Five (16.7%) pregnancies in the study group ended missed abortion vs. none in the control group (P<0.05). Five (20%) threatened aborters delivered between 24(th) and 37(th) weeks of gestation, whereas two (10%) preterm deliveries occurred in the controls (P>0.05). PIBF concentrations in urine (19.5+/-12.9 ng/mL) and serum (214.4+/-120.6 of patients with threatened abortion were significantly lower than in healthy pregnant women (45.3+/-33.7 ng/mL and 357.3+/-159.9 ng/mL, respectively). Women with threatened abortion had significantly lower serum levels of anti-inflammatory cytokine, but levels of proinflammatory cytokines were higher in this group compared with healthy controls. Determination of progesteron-induced blocking factor level in body fluids in early pregnancy might be used for the diagnosis and prognosis of threatened abortion.