Supernumerary kidneys are often asymptomatic and do not affect overall renal function. Thus a significant number of such kidneys may never be discovered or may be noted only as incidental findings during the workup of another unrelated complaint. In some patients, however, supernumerary kidneys present as palpable abdominal masses or cause symptomatic nephrolithiasis or an upper urinary tract infection. Because of the rarity of this condition, affected patients are often not diagnosed until their fourth decade, if at all.