Routine ultrasonographic detection of renal tract dilatations in pregnant women is used to define a high risk population for urinary tract infections. In a first group of 96 patients with symptomatic urinary tract infection, 34% of the cases of uncomplicated infection were accompanied by ultrasonographic abnormalities, while 73% of cases of pyelonephritis presented such signs. Antibody coated bacteria were only found in 12% of uncomplicated urinary tract infections and in 30% of cases of pyelonephritis. In a second group of 273 patients, routine ultrasonography revealed abnormalities in 11.5% of cases. 7.5% of patients had dilatations of the upper renal tract, 2% had renal stones and 2% had occult renal malformations. Dilatations of the upper renal tract were associated with asymptomatic urinary tract infections in 30% of cases. Screening for upper renal tract dilatations is recommended between the 17th and 20th weeks of pregnancy. The presence of such lesions constitutes a risk factor for urinary tract infection and appropriate prophylactic treatment should be prescribed.