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Introduction: Ectopic adrenal tissue in the kidney, including “Ectopic adrenal tissue” and “Adrenal-renal fusion”, is a rare event with a specific behavior which may be difficult to distinguish clinically from renal neoplasms. We performed a systematic review on ectopic adrenal tissue variants reported in the literature underlining its clinical aspects. Methods: Manuscripts which presented a case report or case series of ectopic adrenal tissue in the kidney were included even if published in original articles, reviews, or letters to the editor. A specific search on SCOPUS®, PubMed®, and Web of Science® database was performed. Only English language papers published in a period ranging between August 1991 and April 2020 were considered. Additionally, a case we had at our institution is described, and its characteristics are included. Data on clinical presentation, type of adrenal anomaly, location, anatomopathological and immune-histotype characteristics were collected. Results: We identified 888 manuscripts. Among these 29 were included in this systematic review. Overall, 39 patients with renal adrenal fusion or adrenal ectopia were considered. In most cases, the diagnosis was made incidentally, or following investigation for flank pain, abdominal pain, or endocrinological disorders. CT scan frequently identified a solid vascularized lesion that was difficult to distinguish from renal neoplasm. Adrenal fusion was mostly located at the level of the upper pole. Adrenal rest was found in the renal parenchyma, renal hilum, or retroperitoneum in close proximity to the renal peduncle. Often these ectopic adrenal tissue lesions follow a benign behavior and can be classified as functioning or non-functioning adenomas. Rarely, they may experience neoplastic degeneration. The most frequently positive markers were inhibin, vimentin, melan-A, synaptophysin and anti-p450 scc. Conclusions: Ectopic adrenal tissue in the kidney is a rare event with specific clinical characteristics that need to be identified in order to arrive at a correct diagnosis and carry out appropriate treatment management.