We discuss data on urinary tract dilatations diagnosed in utero from February 1980 to May 1988. Over 8,900 pregnancies prenatally investigated with ultrasound during second and third trimester in our center. We diagnosed 401 fetal defects (4.5%) with 121 urinary system anomalies (1.35%). In 90 cases (1.01 %) urinary tract dilatation was present, representing the most frequent urinary pathology in utero (74.3% of all urinary anomalies). We describe some diagnostic findings as particularly useful in differential diagnosis of functional and obstructive conditions, stressing the possibility and the difficulty of diagnostic accuracy for urinary tract anomalies. The perinatal outcome of each condition is reported.