We studied a human embryo of 16 mm crown-rump (CR) length in excellent condition with a horseshoe kidney malformation. An exhaustive study of this specimen and a review of published material on the human embryo brings us to propose a new theory on the embryogenesis of this malformation. The most commonly accepted theory consists of a mechanical interpretation based on the relation between the metanephroi and the umbilical arteries during the development of the latter. Nevertheless, in those cases where renal parenchyma constitutes the isthmic region, we believe that these arise from nephrogenic cells that have migrated across the primitive streak in the final phase of gastrulation and thus arise from the posterior nephrogenic area of the epiblast.