A case of retroperitoneal neurofibroma associated with von Recklinghausen's disease is reported, and has never been recorded in Japanese medical papers in the past. A 46-year-old female, who had many café-au-lait spots on the trunk and was regarded as a patient with von Recklinghausen's disease, was noticed to have a hand-grip-sized tumor at the left and lower abdomen. She felt numb at the left leg when the tumor was strongly compressed. Ultrasonographically the tumor was an oval-shaped hypoechoic mass with a clear border, and contained a central hyperechoic region, and also cross-shaped hyperechoic lines(that is, a bundle echo different from the septum echo) which seemed to suggest the nerve bundle. CT of the tumor showed a clearly demarcated mass between the left major psoas muscle and the pelvic bone in the retroperitoneal space. MRI of the retroperitoneal tumor indicated a neurogenic tumor originating in the ischiac nerve. In angiography the neurogenic retroperitoneal tumor was found to be nourished by the parasitic vessel from the internal ilial artery. In operative findings the tumor was confirmed to be a neurogenic retroperitoneal type originating in the ischiac nerve. Pathohistology of a part of the tumor showed neurofibroma without malignancy. Neurofibroma in a patient with von Recklinghausen's disease might become malignant, and is occasionally difficult to differentiate from malignant Schwannoma pathohistologically. This patient's prognosis was carefully observed to see whether or not the neurofibroma changed to be a malignant schwannoma.