Supernumerary kidney.

We studied 58 case reports of patients with supernumerary kidneys to determine the morphology, vagaries and embryogenesis of this rare and poorly documented anomaly. The supernumerary kidney usually was located caudal to the ipsilateral kidney when subserved by a bifid ureter and cranially when the ureters were separate. The Weigert-Meyer law for duplex fused kidneys was obeyed by the supernumerary ureter in most fully documented cases of double ureters. Pathologic conditions of the upper urinary tract occurred in more than 50 per cent of the patients with a bifid system, who were prone to have hydronephrosis and calculous disease, and with a double system, who were prone to have complications resulting from supernumerary ureteral ectopia. Double tails to the nephrogenic cords, each induced by a branch of a bifid bud or by 1 of 2 separate buds as opposed to tandem inductions of a single metanephros, were regarded as the probable embryogenesis.