Background: Preterm birth is a leading cause of perinatal mortality and long-term morbidity as well as the long-term health consequences and cognitive outcomes.Objective: Present study was conducted to determine prevalence and risk factors associated with preterm birth in Ardabil, Iran.Materials and Methods: A case control study was conducted between Nov 2010 and July 2011 in all three maternal hospitals in Ardabil. All the live newborns during the study period were investigated. Of 6705 live births during the study period 346 births occurred in <37 weeks were taken as a case and 589 term neonates were taken as a control group. Data were obtained through review of prenatal and hospital delivery records. Univariate and multivariate logistic regression analysis were applied to obtain magnitude of association between independent variables and preterm birth.Results: The prevalence rate of preterm birth was 5.1%. History of previous preterm birth (OR=12.7,CI: 3.9-40.4, p<0.001), hypertension (OR=7.3, CI:2.1-25.4, p=0.002), Oligohydramnios (OR=3.9, CI:1.6-9.5, p=0.002), spouse abuse (OR=3.7, CI:1.1-11.8, p=0.024), preeclampsia (OR=3.6, CI:1.3-10.3, p=0.014), premature rupture of membrane (OR=3.1, CI:1.9-4.9, p=0.000), bleeding or spotting during pregnancy (OR=2.0, CI:1.0-3.8, p=0.037), Hyperemesis Gravid arum (OR=2.0, CI: 1.1-3.8, p=0.015), urinary tract infection in 26-30 weeks, (OR=1. 8 CI:1.0-3.2, p=0.04), diastolic blood pressure £60 mmg (OR=1.5, CI: 0.99-2.2, p=0.049) were determined as significant risk factors for preterm birth.Conclusion: Early detection and treatment of diseases or disorders among pregnant women especially hypertension, Oligohydramnios, preeclampsia, bleeding or spotting, Hyperemesis Gravid arum, urinary tract infection, and low diastolic blood pressure as well as the improving health care quality delivered to pregnant women may reduce preterm prevalence rate.