A cohort study with four groups of pregnant women was designed to evaluate if a decrease in the concentration of fibronectin (FN) in amniotic fluid plays a role in the genesis of premature rupture of the membranes (PROM) and/or intra-amniotic infection: group 1, 15 patients with PROM and intra-amniotic infection; group 2, 15 patients with PROM but without intra-amniotic infection; group 3, 15 patients with preterm labor without PROM or intra-amniotic infection; and group 4, 15 patients who underwent elective amniocentesis for fetal lung maturity studies. No significant differences in amniotic fluid concentrations of FN among any of the four groups was observed (F = 0.146; P = 0.931). These results suggest that a deficiency of FN in amniotic fluid is not a critical factor in determining which patients may be affected by PROM or which of them will develop an intra-amniotic infection.