Persons with horseshoe kidneys are at higher risk for renal calculi than the average population (~20% lifetime risk). Because of the ectopic exit of the ureters from the horseshoe kidney, the increased risk of stones has been thought to be due to urine stasis rather than abnormalities of the renal parenchyma. Some think that a combination of relative urinary stasis and metabolic abnormalities in the horseshoe kidney are required for stone formation.