Patients were identified using ICD-9-CM codes for preterm labor and delivery. The medical record was reviewed to determine exposure to MgSO4 as a tocolytic agent, excluding patients who received MgSO4 for other indications. Cases were defined as probable when patients met the clinical definition for pulmonary edema, including chest X-ray findings. The medical charts of both cases and a random sample of non cases were then reviewed in detail and the data from these groups were compared. The primary analysis of these case-control data included unadjusted analysis and multivariable explanatory models to provide insight into the risk factors for pulmonary edema associated with MgSO4. The data from the case-control study were used to develop a clinical predictive index. The predictive rule was then validated in a separate patient population.