OBJECTIVE: This study was undertaken to determine whether the interleukin-1 receptor antagonist (IL-1RN) variable number tandem repeat polymorphism is associated with preterm birth. STUDY DESIGN: A case-control study was performed. Cases (n = 95) delivered before 37 weeks after preterm labor (PTL) or preterm premature rupture of membranes (PPROM) and controls (n = 105) delivered after 37 weeks. Maternal DNA was genotyped by polymerase chain reaction for a length polymorphism in intron 2 of the IL-1RN gene. RESULTS: There was no significant difference in maternal age, ethnicity, insurance status, or parity between groups. Allele and genotype frequencies did not differ significantly from that expected under Hardy-Weinberg equilibrium (P = .59) in the total group as well as study groups. Of the 95 cases, 26.8% had at least 1 copy of allele 2 present compared with 12.4% in the control group (P < .0004). CONCLUSION: Maternal carriage of at least 1 copy of the IL-1RN allele 2 appears to be associated with increased risk of preterm birth.