[Diagnosis of an opacity at the lung base: intrathoracic kidney]

A 72-year-old patient was admitted for exploration of an opacity of the left base discovered fortuitously on a routine chest x-ray. Initial blood tests were normal. Fiberoptic bronchoscopy was normal. Computed tomography (CT) led to the diagnosis of a left kidney which had risen into a retro-cardiac position. Magnetic resonance imaging established the sub-diaphragmatic position of the kidney. Renal excretion was normal on intravenous urography. An ectopic kidney in an intrathoracic position is very uncommon and may raise a major challenge when visualized as a mediastinal or pulmonary opacity. Computed tomography or intravenous urography can provide the diagnosis and magnetic resonance imaging demonstrates its precise sub-diaphragmatic or supra-diaphragmatic position.