Ectopic supernumerary kidney is a rare congenital urinary tract abnormality. Because of the scarcity of published cases and atypical presenting symptomatology this entity frequently causes a diagnostic as well as therapeutic dilemma. We report a case of an unusually symptomatic supernumerary kidney that presented as back pain. Noncontrast CT scan showed a suspicious left-sided para-aortic mass, which prompted a percutaneous biopsy. Intravenous contrast CT scan revealed an anatomically and functionally free supernumerary kidney. The approach to diagnosis as well as management of supernumerary kidneys is discussed herein.