Atypical neurofibromatous neoplasm of uncertain biological potential (ANNUBP), defined as a borderline lesion that is difficult to distinguish whether benign or malignant, is one of the intermediate stages to malignant peripheral nerve sheath tumor, a peripheral nerve-derived malignant tumor that develops from nerve sheath cells. Because ANNUBP is a new concept, only a few cases have been reported, all in patients with neurofibromatosis type 1 (NF-1).An 88-year-old woman presented with a mass on the left upper arm persisting for one year. Magnetic resonance imaging showed a large tumor spreading between the humerus and biceps muscle, which was diagnosed as undifferentiated pleomorphic sarcoma by needle biopsy. Extensive tumor resection was performed, including partial cortical bone resection of the humerus. Based on the histological features, although the patient did not have NF-1, the tumor was strongly suspected to be ANNUBP. As malignant peripheral nerve sheath tumors have been sporadically reported in patients without NF-1, it is feasible that ANNUBP could also occur in patients without NF-1.