OBJECTIVE: To test the effects of maternal periodontal disease treatment on the incidence of preterm birth (delivery before 37 weeks of gestation). METHODS: The Maternal Oral Therapy to Reduce Obstetric Risk Study was a randomized, treatment-masked, controlled clinical trial of pregnant women with periodontal disease who were receiving standard obstetric care. Participants were assigned to either a periodontal treatment arm, consisting of scaling and root planing early in the second trimester, or a delayed treatment arm that provided periodontal care after delivery. Pregnancy and maternal periodontal status were followed to delivery and neonatal outcomes until discharge. The primary outcome (gestational age less than 37 weeks) and the secondary outcome (gestational age less than 35 weeks) were analyzed using a χ test of equality of two proportions. RESULTS: The study randomized 1,806 patients at three performance sites and completed 1,760 evaluable patients. At baseline, there were no differences comparing the treatment and control arms for any of the periodontal or obstetric measures. The rate of preterm delivery for the treatment group was 13.1% and 11.5% for the control group (P=.316). There were no significant differences when comparing women in the treatment group with those in the control group with regard to the adverse event rate or the major obstetric and neonatal outcomes. CONCLUSION: Periodontal therapy did not reduce the incidence of preterm delivery. CLINICAL TRIAL REGISTRATION: ClinicalTrials.gov, www.clinicaltrials.gov, NCT00097656. © 2009 The American College of Obstetricians and Gynecologists.