Pseudohydronephrosis on bone scan due to crossed renal ectopia.

We present a patient with crossed renal ectopia whose bone scan gave the appearance of unilateral hydronephrosis with contralateral absent or nonfunctioning kidney. An excretory urogram revealed crossed renal ectopia without obstruction. The axiom that unilateral absence of a kidney without previous nephrectomy should stimulate a search for ectopic kidney applies equally well to the bone scan as to the excretory urogram.