Thirty-seven consecutive patients with singleton pregnancies in "uncomplicated" preterm labor with intact membranes suitable for tocolysis were evaluated for evidence of silent chorioamnionitis by means of maternal serum C-reactive protein and amniotic fluid white blood cell count, Gram stain, and cultures. Abnormalities in these markers of infection were found to be significantly more common in cases that were refractory to tocolysis. These cases also showed both pathologic evidence of chorioamnionitis and a significantly greater neonatal early infectious morbidity. We conclude that silent chorioamnionitis is a significant cause of "uncomplicated" preterm labor refractory to conventional methods of tocolysis.