Abstract
A woman in her 70s presented with severe abdominal pain following a mechanical fall. A CT of the abdomen revealed a massive cystic lesion with evidence of rupture, while MRI localised the lesion to the left retroperitoneum; however, its aetiology remained unclear. She underwent laparotomy with extensive adhesiolysis and excision of a large multiloculated retroperitoneal cyst containing approximately 6500 mL of serous and haemorrhagic fluid. Histopathological examination demonstrated a benign serous cystadenoma lined by ciliated serous epithelium with tubal metaplasia and no evidence of atypia. This case highlights the diagnostic challenges associated with primary retroperitoneal serous cystadenomas and emphasises the importance of surgical excision and histopathological assessment for definitive diagnosis.