OBJECTIVE To determine whether the levels of the cytokines interleukin-6 (IL-6) and IL-8 in amniotic fluid identify patients with preterm labor who are resistant to tocolysis. METHODS Amniocenteses were performed in 23 women with documented preterm labor at 20-32 weeks' gestational age who were treated subsequently with tocolytics. The concentrations of IL-6 and IL-8 in amniotic fluid were determined by double-antibody radioimmunoassay methods using recombinant human IL standards. RESULTS Of the 23 patients, five failed to respond to tocolysis (four delivered within 48 hours), and of the remaining 18, all delivered more than 9 days after tocolysis was initiated (mean 31 +/- 10 days; range 9-61). In women who had failed tocolysis, discriminatory concentrations of IL-6 and IL-8 were 20 and 15 ng/mL, respectively. Of the patients who had amniotic fluid concentrations higher than these thresholds, all failed tocolysis (100% positive predictive value) and delivered within 6 days. The patients with levels below these discriminatory concentrations had successful tocolysis, and 17 of 18 delivered more than 2 weeks after treatment (95% negative predictive value). CONCLUSIONS The success of tocolysis and thus delivery remote from an episode of preterm labor is associated with discriminatory amniotic fluid IL-6 and IL-8 levels of less than 20 and less than 15 ng/mL, respectively. If the immunologic response that causes the release of IL-6 and IL-8 has not occurred, the likelihood of successful tocolysis is extremely high. However, if both IL-6 and IL-8 are elevated, tocolysis is likely to fail and delivery may occur within 48 hours.