Leiomyoma is the most common benign solid pelvic tumor seen in women. It is most commonly located in the uterus and gastrointestinal tract, but it can originate wherever smooth muscle cells exist. Although it has been reported in various atypical localizations, they are extremely rare in the retroperitoneum. Also, preoperative diagnosis is often difficult in retroperitoneal tumors. Imaging studies may demonstrate the retroperitoneal tumors; however, exact diagnosis cannot be established by imaging methods alone. Here, we report a case with retroperitoneal leiomyomatosis diagnosed by preoperative ultrasonography-guided fine-needle biopsy.