During a 6-year period (1979-1985), 142 neonates with significant hydronephrosis were seen. Seventy-eight percent of the cases were discovered on fetal screening during obstetric sonography. Maternal/fetal intervention was virtually never indicated and most babies were asymptomatic. The most common conditions found were obstruction of the ureteropelvic junction (41%), obstruction of the distal ureter (usually primary megaureter) (23%), upper-pole hydronephrosis associated with duplex anomalies (13%), and posterior urethral valves (10%). Seventeen neonates with multicystic dysplastic kidney were seen (three per year or one for every eight with hydronephrosis). In comparison, during the 30-year period, 1947-1977, 146 neonates with significant hydronephrosis were seen. Most cases were discovered because the patients had signs and/or symptoms--either an abdominal mass (an enlarged kidney or bladder) or urosepsis. The three most common conditions were obstruction of the ureteropelvic junction (22%), posterior urethral valves (19%), and ectopic ureterocele (14%). During this period, 53 neonates with multicystic dysplastic kidney were discovered (two per year or one for every three with hydronephrosis). The dramatic increase in the number of neonates found to have hydronephrosis is primarily due to the widespread use of obstetric sonography and concomitant fetal screening. The pattern of causes before 1979 represented the incidence of symptomatic lesions. The current pattern more accurately reflects the true incidence of congenital anomalies of the urinary tract.