The development of pulmonary injury in cases of antepartum pyelonephritis is rare but serious. To date, factors that might identify patients at risk have not been determined. We compared 11 patients with pyelonephritis and pulmonary injury with 119 patients with pyelonephritis only. Pulmonary injury was more likely to occur in the more severe cases; however, the presence of a maternal heart rate greater than 110 beats/min and a fever to 103 degrees F 12 to 24 hours before the occurrence of respiratory symptoms in a gestation greater than 20 weeks was highly predictive of pulmonary injury. The most significant predictive factors associated with pulmonary injury were elements of treatment such as fluid overload, use of tocolytic agents, and, to a lesser extent, choice of antibiotic. Therefore, if tocolytic agents are considered at all in the management of acute pyelonephritis in pregnancy, they should be used only in patients with documented cervical change. In addition, urinary output should be monitored very closely. These data also may suggest a cause of the pulmonary edema that is occasionally seen in the management of premature labor with the use of tocolytic agents and fluids in the presence of a possible occult infection.