OBJECTIVE To measure the amniotic fluid (AF) and plasma concentrations of cortisol in women with preterm labor and intact membranes. METHODS Thirty-eight normal healthy women with singleton gestations, preterm labor, and intact membranes at 32-36 weeks' gestation underwent amniocentesis for evaluation of the microbiologic status of the amniotic cavity. Nineteen women delivered within 1 week of amniocentesis (preterm labor and delivery group), and 19 delivered at term (preterm labor and term delivery group). Maternal plasma and AF concentrations of cortisol were measured with sensitive and specific, commercially available radioimmunoassay kits. RESULTS The median plasma cortisol concentration was significantly higher in women who delivered prematurely than in those who delivered at term (260 versus 240 ng/mL; P = .014). However, no significant differences in median AF cortisol concentrations were detected between the groups (13 versus 14 ng/mL). CONCLUSIONS Although maternal plasma cortisol concentrations were significantly higher in women with preterm birth, no similar changes were found in the AF. The rise in maternal plasma cortisol may be related to the stress mechanism of labor.