The ureters carry urine from the kidneys to the bladder. Each kidney has its own ureter. A ureterocele involves a widening of the lower part of the ureter, where it enters the bladder, due to a blockage caused by a membrane. Because urine cannot flow freely into the bladder, this membrane stretches out and fills with urine, resembling a water balloon.

Ureteroceles occur in about 1 in 500 to 1 in 4,000 people. Caucasians are most likely to be affected. It is equally common in both left and right ureters.

The defect occurs during fetal development. The size of the ureterocele determines when it will be diagnosed; large ureteroceles are generally discovered earlier in life than smaller ones.

Ureteroceles may be discovered before birth during a prenatal maternal ultrasound (pregnancy ultrasound). Children with this condition often come to medical attention for severe urinary tract infections. Sometimes people with ureteroceles don't know they have the condition, or the diagnosis is made later in life due to kidney stones. Uteroceles can also cause chronic unilateral (one-sided) or bilateral (both-sided) obstructive uropathy (swelling) of the affected kidney(s).

Uteroceles can be complicated by incorrect placement of the ureter (ectopic ureter) or other congenital anomalies, including ureteropelvic junction obstruction (blockage of the connection between the kidney and the ureter) or vesicoureteral reflux (backflow of urine from the bladder into the ureter) on the opposite side.