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Ovarian-type mucinous tumours occur very rarely in the retroperitoneum. We present a case of primary retroperitoneal mucinous tumour of borderline malignancy in a 58-year-old woman, detected as an incidental finding. The patient presented with acute renal failure, investigation for which revealed a complex pelvic mass initially thought to be in the right adnexa and consistent with an ovarian neoplasm. Surgical findings revealed a 130-mm, right-sided non-communicating retroperitoneal pelvic mass, posterior to the appendix, which was completely resected. Both ovaries were normal. Macroscopically, it was a multi-loculated cystic structure with a smooth external surface containing clear and mucinous fluid. Microscopic examination showed a mucinous tumour of borderline malignancy. The literature contains approximately nine other cases of primary mucinous retroperitoneal tumour of borderline malignancy. These cases have occurred in women aged 36–60 years. Most patients were asymptomatic and the mass was detected as an incidental finding. The patients have been followed up for up to 6 to 18 months and, to date, none have recurred. There are limitations to pre-operative radiological imaging. A definitive diagnosis can only be made after complete surgical excision and histological examination, having excluded retroperitoneal involvement by mucinous tumours from sites such the ovaries, bowel, appendix and pancreas.