With advances in obstetric ultrasound, prenatal observation of the pathogenesis and natural history of obstructive uropathy is possible. The Authors describe the ultrasonographic appearance of a fetal monolateral urinoma associated with a severe hydronephrosis and the progressive reduction of the urinoma during the last weeks of the pregnancy until complete disappearance in the first month of life. Urinoma detected antenatally, with onset very early in gestation, usually is associated with severe urinary tract obstruction, like posterior urethral valves or ureteropelvic junction obstruction and is always associated with a severe impairment of ipsilateral renal function.