A case of ureterocele diagnosed by an experimental trial using transvaginal ultrasound is reported. Color Doppler sonography revealed a jet flowing from the ureterocele into bladder. The radionuclide renal scan revealed a right duplex system, a ureterocele of the upper moiety and its functional obstruction. If the upper moiety was obstructed, where was the ureteral jet from? We can suppose that the jet visualized was flowing from the ureteral meatus of the lower moiety, positioned behind the ureterocele, and not from the ureterocele itself. In the light of this case, a urinary obstruction could not be excluded only on the basis of Doppler sonographic findings. In conclusion, a diagnostic work-up using only transvaginal ultrasound is insufficient to adeguately analyse the anatomy and function of the urinary system, especially when there are anatomy malformations.