Unilateral agenesia has an incidence of 1/1000, occurs more often in males and the majority are on the left side. They often go unnoticed because they are compatible with a normal life. One observes a compensatory hypertrophy of the normal kidney (> as 10% of the normal renal volume).

This congenital anomaly is extremely seldom. Even if a doubling of the urinary tract occurs frequently, that of the kidneys is extraordinarily rare. Involved is an independent kidney with its own vascular supply, a capsule and its own urinary tract. The cause is a very early division of the ureter anlage before the invasion into the metanephric blastema.

This anomaly is relatively frequent. One speaks of a malrotation if the pyelo-ureteral connection is oriented ventrally (missing rotation), dorsally (rotation of more than 90°) or laterally (inverse rotation). The kidney and urinary tract are here anatomically and histologically normal. Sometimes, though, one observes anomalies in the connections between ureter and renal pelvis, which can lead to a hydronephrosis (enlargement of the renal pelvis, the major and minor calices, leading in turn to an enlarged kidney).