OBJECTIVE: To assess the effectiveness of cervical pessary in reducing the rate of preterm birth in women at high risk for preterm birth who did not deliver after an episode of threatened preterm labor. METHODS: In a multicenter open-label randomized controlled trial, a cervical pessary was compared with no intervention (control group) (one-to-one ratio). Women between 24 and 34 weeks of gestation at high risk for preterm birth based on a short cervical length (less than 15 mm) or an intermediate cervical length (between 15 and 30 mm) with a positive fetal fibronectin test who did not deliver after an episode of threatened preterm labor were eligible. The primary outcome was birth before 37 weeks of gestation. Secondary outcomes were a composite adverse neonatal outcome, preterm birth before 34 and 32 weeks of gestation, and side effects. A total sample size of 200 women carrying singletons was planned so as to have adequate statistical power to detect a reduction in the rate of preterm birth from 40% to 20%. Women with twin gestations were also enrolled but were considered only in secondary analyses. After a planned interim analysis, the trial was stopped for futility. RESULTS: From November 2013 through September 2016, 130 women with a singleton pregnancy (65 pessary, 65 no treatment) were recruited. The groups had comparable baseline characteristics. In the cervical pessary group, 31 (48%) women delivered before 37 weeks of gestation compared with 25 (39%) in the no-treatment group (relative risk 1.2, 95% CI 0.83-1.8). Nine (15%) children in the cervical pessary group had the composite adverse perinatal outcome compared with eight (13%) in the control group (relative risk 1.2, 95% CI 0.49-2.9). CONCLUSION: In women at high risk for preterm birth who did not deliver after an episode of threatened preterm labor, treatment with a cervical pessary is not effective.