Although prostaglandins (PGs) are considered the key mediators of human parturition at term, there is a paucity of data regarding their participation in the mechanisms responsible for preterm labor. The purpose of this study was to establish if preterm labor is associated with changes in the amniotic fluid concentrations of prostaglandins. PGF2α, 13,14-dihydro-15-keto-prostaglandin F2α (PGFM) and 11-deoxy-13,14-dihydro-15-keto-11,16-cyclo-prostaglandin E2 (PGEM-II) were measured by using specific and sensitive radioimmunoassays. Amniotic fluid was retrieved by transabdominal amniocentesis from 55 women with preterm labor and intact membranes. Patients were divided into three groups according to the response to tocolysis and the presence or absence of an intra-amniotic infection. Amniotic fluid concentrations of PGFM and PGEM-II were significantly greater in women with preterm labor and intra-amniotic infection than in women without infection. In addition, patients unresponsive to tocolysis without intra-amniotic infection also had a significantly greater concentration of PGFM and PGEM-II in amniotic fluid than those responsive to tocolysis. Amniotic fluid concentrations of PGF2α were greater in women with intra-amniotic infection than in women without intra-amniotic infection. In the absence of intra-amniotic infection, no difference in amniotic fluid PGF2α concentrations could be found between women who responded to tocolytic treatment and those who did not.