A 54-year-old asymptomatic woman was first diagnosed with unicentric Castleman's disease at 51 years of age. The disease was identified incidentally behind the inferior vena cava after a computed tomography (CT) scan. She was followed conservatively and no enlargement of the mass was observed. However, a follow-up CT scan revealed another solid mass in the left retroperitoneal area. Both tumors were surgically removed in the same session. The retrocaval mass was histologically confirmed to be Castleman's disease of the hyaline vascular type. Histological examination of the left peritoneal mass revealed low-grade leiomyosarcoma. We present here a case of leiomyosarcoma associated with unicentric Castleman's disease, both of which were localized in the retroperitoneum.