Primary retroperitoneal leiomyomas are a very rare clinical condition. We present the case of degenerated primary leiomyoma in retroperitoneal area. A 33-year-old primiparous woman presented with chronic constipation. On examination, there was a large firm unpainful palpable lumbar mass in the left side. The diagnostic workup included contrast computer tomography, transabdominal ultrasonography, and colonoscopy that revealed a 7,5-cm heterogeneous retroperitoneal tumor in the left infrarenal area. At laparotomy, the mass arising from the retroperitoneum was excised. Histopathology showed a degenerated leiomyoma. Preoperative imaging can only give provisional diagnosis and can be misguiding. Considering current limitations in radiological diagnosis, surgical excision of these tumors is necessary to rule out malignancy.