Twenty-six fetuses with dilated urinary tracts were studied with serial sonograms. Eight fetuses with unilateral hydronephrosis were followed without intervention; all are well after postnatal surgical correction. Three cases of bilateral hydronephrosis resolved spontaneously before birth. Eight fetuses with bilateral hydronephrosis had evidence of poor function: Three were not treated and died shortly after birth with small lungs and dysplastic kidneys; three others had diagnostic intervention that demonstrated irreversible disease and allowed termination of the pregnancy; two had obstruction successfully corrected in utero, but renal damage proved irreversible and precluded survival at birth. Seven fetuses with bilateral hydronephrosis and equivocal function underwent early decompression. Four were delivered early and corrected ex utero; 1 has renal failure and the other 3 are well. Three had obstruction relieved in utero by a catheter shunt placed percutaneously; 1 had multiple anomalies and died; the other 2 are well. Serial sonographic observation improves perinatal management of the fetus with a dilated urinary tract. The need for diagnostic or therapeutic intervention depends on the type and severity of obstruction and the time in gestation when it is discovered. Most fetuses do not require treatment before birth; a few may benefit from early decompression in or ex utero.