We report on 3 patients with tumor in a horseshoe kidney, 1 of whom had bilateral tumor (renal cell cancer on the right side and urothelial cancer on the left side). Tumors that arise predominantly in the bridge of a horseshoe kidney can mimic the symptoms of an intra-abdominal disease process. Besides routine diagnostic procedures, angiography is essential to plan the surgical approach, which in principle should be organ-sparing. The literature of the embryology of the horseshoe kidney was reviewed for a relationship between the abnormal renal development and the site of tumorigenesis, and for development of a key for the wide variation of blood supply. Recently reported data suggest that the theory of a mechanical fusion is valid only for horseshoe kidneys with a fibrous isthmus but that an abnormal migration of the posterior nephrogenic area causes the majority of horseshoe kidneys in which the isthmus consists of parenchyma. Development of the isthmus through abnormal migration could predispose this location for renal cell cancer and would explain the varying forms of blood supply. Additionally, this hypothesis supports the previously raised assumption that horseshoe kidneys may be the result of teratogenic factors, which also may be responsible for the known increased incidence of related congenital anomalies and of nephroblastoma.