An ectopic kidney is a congenital anomaly characterized by the disruption of kidney migration during embryologic development. This results in failure of the kidney to ascend and rotate to its normal position. Ectopic kidneys are most commonly found in the pelvis. Rarely, the kidney may migrate to the thoracic position.

There are several subtypes of renal ectopia. Simple renal ectopia describes a kidney that is positioned ipsilateral to where its ureter inserts into the bladder. Crossed renal ectopia describes a kidney that has crossed the midline. A crossed ectopic kidney may fuse with the normally positioned kidney, creating an L-shaped kidney. Renal ectopia may also be associated with fusion anomalies. An example of this is the horseshoe kidney, which describes the fusion of the kidneys' inferior poles and failure to ascend past the inferior mesenteric artery.

Renal ectopia has an estimated incidence of 1 in 5000 patients. Renal ectopia and horseshoe kidneys are usually isolated anomalies. Most cases are asymptomatic and found incidentally during imaging for other conditions. Thus, diagnosis may occur at any age. However, Turner syndrome and trisomies 13, 18, and 21 are associated with the presence of a horseshoe kidney.

Ectopic kidneys are associated with increased risk for renal complications including nephrolithiasis, vesicoureteral reflux, ureteropelvic junction obstruction, hydronephrosis, extrarenal calyces, renal infections, and certain malignancies. The most common malignancy in horseshoe kidneys is renal cell carcinoma (RCC), but the incidence of RCC is similar to that of the general population. There are increased incidences of Wilms tumor, carcinoid tumor, and transitional cell tumors with these renal anomalies, but these tumors are still less common than RCC.