Supernumerary kidney: typical and atypical features.

The clinical and radiologic features of three children with accessory renal units are presented. On the basis of the pyelographic and ultrasound findings, the first two clearly had free supernumerary kidneys. The third had a more complex "supernumerary variant," wherein the accessory renal unit formed the isthmus of a horseshoe or "pseudohorseshoe" kidney.