Ureteropelvic junction (UPJ) obstruction is being detected with increasing frequency before birth. To clarify the natural history of fetal UPJ obstruction, we reviewed our experience managing 28 fetuses; there were 16 bilateral cases for a total of 44 renal units. None required decompression before birth. We found that fetal bilateral UPJ obstruction is associated with significant morbidity and mortality; resolution of fetal hydronephrosis secondary to UPJ obstruction is rare; antenatal diagnosis of UPJ obstruction improves clinical management by allowing early detection and appropriate treatment of otherwise clinically undetectable disease; oligohydramnios in the mature fetus with bilateral UPJ obstruction is an indication for early delivery and immediate repair; and prenatally diagnosed UPJ obstructions should be repaired as early as possible after birth.