Objective: The objective of the study was to assess cerclage to prevent recurrent preterm birth in women with short cervix. Study Design: Women with prior spontaneous preterm birth less than 34 weeks were screened for short cervix and randomly assigned to cerclage if cervical length was less than 25 mm. Results: Of 1014 women screened, 302 were randomized; 42% of women not assigned and 32% of those assigned to cerclage delivered less than 35 weeks (P = .09). In planned analyses, birth less than 24 weeks (P = .03) and perinatal mortality (P = .046) were less frequent in the cerclage group. There was a significant interaction between cervical length and cerclage. Birth less than 35 weeks (P = .006) was reduced in the less than 15 mm stratum with a null effect in the 15-24 mm stratum. Conclusion: In women with a prior spontaneous preterm birth less than 34 weeks and cervical length less than 25 mm, cerclage reduced previable birth and perinatal mortality but did not prevent birth less than 35 weeks, unless cervical length was less than 15 mm. © 2009 Mosby, Inc. All rights reserved.