Background: Giant retroperitoneal cysts are uncommon, with an estimated incidence of 1/5750 to 1/250,000. Degenerative changes of leiomyoma may lead to complex presentation resulting in misdiagnosis. The final diagnosis can be made either during surgery or histologically. The most prevalent mesenchymal tumors at this site are of a lipomatous, myogenic or neuroectodermal nature. Case Report: We have a case of large leiomyoma of retroperitoneum with cystic degeneration confirmed on immune-histochemistry, which presented to us as right iliac fossa herniation masquerading as a huge right ovarian cyst. Conclusion: The retroperitoneal leiomyoma should be considered in the differential diagnosis of a giant cystic mass in abdomen.