The identification of risk factors for intra-amniotic infection may allow changes in obstetric management and reduce complications. In a pilot study, stepwise logistic regression identified duration of ruptured membranes and duration of interval monitoring as significant risk factors for intra-amniotic infection. Using the preliminary regression equation, we established critical durations (risk criteria) to predict a 20% or greater probability of intra-amniotic infection. Subsequently, 2908 patients were screened prospectively for risk criteria and/or the presence of intra-amniotic infection. Seven hundred five patients of the 2908 (24%) met the risk criteria, and 107 of 124 cases of intra-amniotic infection (86%) met the risk criteria. In patients meeting the risk criteria, the relative risk of intra-amniotic infection was 19.7. In addition, 81 of 705 (12%) of mothers developed endometritis. Ten neonates of mothers with criteria developed sepsis or pneumonia. A stepwise logistic regression performed on the prospectively gathered data showed that among patients meeting risk criteria, parity, duration of internal monitoring, and duration of membrane rupture were the significant risk factors for intra-amniotic infection.