Objective: To assess whether type of suture material affects cerclage efficacy for preterm birth (PTB) prevention. Methods: Secondary analysis of a multicenter trial of ultrasound-indicated cerclage for short cervical length (CL), in which women with prior spontaneous PTB at 1633 6/7 weeks, a singleton gestation and CL < 25 mm between 1622 6/7 weeks, were randomized to McDonald cerclage or no cerclage. Outcomes of women who underwent cerclage were analyzed by type of suture material, comparing polyester braided thread (Mersilene™ or Ethibond™) to Mersilene tape™. Primary outcome was PTB < 35 weeks. Results: 138 women underwent McDonald cerclage: 84 (61%) received polyester braided thread and 46 (33%) Mersilene tape™. Eight (6%) received monofilament suture and were excluded from analysis. Rates of PTB < 35 weeks were similar, 35% for polyester braided thread vs 24% for Mersilene tape™ (p = .24). Birth gestational age was also similar among the 2 groups (p = .18). Conclusion: Type of suture material may not affect ultrasound-indicated cerclage efficacy in high-risk women with short CL, but further study is needed. Polyester braided thread (Mersilene™ or Ethibond™) and polyester braided Mersilene tape™ seem to have similar efficacy. © 2012 Informa UK, Ltd.