In paediatric radiological practice, the diagnosis of either horseshoe kidney or Wilm's tumour is not particularly uncommon, but their discovery in combination is extremely rare. In the horseshoe type of renal fusion, the two kidneys are joined across the mid-line by an isthmus formed from the lower poles, usually at about the level of L.3 or L.4. The kidney pelvis is anterior to the blood supply, which consists of four to six arteries. The ureters run anterior to the isthmus and parallel to the lumbar spine. The frequency of horseshoe kidney at autopsy is about one per 1,000 but pyelography shows it more frequently, about once in 400 to 500 examinations (Smith, 1946). The anatomy of the abnormality is such that the fused kidneys are more prone to hydronephrosis due to pelvi-ureteric obstruction from kinking by vessels or bands; occasionally there is direct ureteric compression at the isthmus (Higgins, Williams and Nash, 1951). Calculi and infection are also common (Gutierrez, 1932). Nephroblastoma or Wil...