On the basis of a case of thoracic ectopic kidney, discovered following trauma and diagnosed at laparotomy (since it was confused with rupture of the left diaphragm) the authors review certain data concerning this rare malformation (150 published cases). --The site of origin of the renal artery may be used to distinguish on the one hand between true intra renal ectopic kidney with the renal artery arising from the aorta and secondly trans-diaphragmatic herniation of a kidney with a kidney with a normal vascular pedicle through a hernia, most often congenital, and equally often right as left. --Clinically, at the time of a routine examination or following trauma, chest X-ray reveals an opacity at the base of the thorax, the low and posterior topography of which is suggestive of the diagnosis and and IVP is then indicated. In the absence of violent trauma, the diagnosis of traumatic hernia is eliminated. The distinction between true ectopic kidney and congenital trans-diaphragmatic hernia becomes a false problem since there is no need for treatment in either case.