Low-grade intraosseous osteosarcoma is an uncommon form of bone cancer. It is occasionally difficult to recognize as a malignant tumor and is commonly misdiagnosed as a benign fibrous lesion. We retrospectively studied the records of 8 patients with low-grade intraosseous osteosarcoma in the files of the Tohoku Musculoskeletal Tumor Society in Japan. All tumors arose in the lower limb. The most common symptom was pain, with a duration exceeding 2 years in 4 patients. Radiologic findings, including those at magnetic resonance imaging (MRI), suggested malignancy in 5 lesions, whereas 3 were diagnosed as benign. Two patients initially presented with pathological fracture. The initial pathological diagnosis was malignant in 5 patients and benign in 3. All eight tumors were grade 1 in Broders' classification. The tumor showed a permeative pattern in all eight cases, but this pattern could not be confirmed in the multiple tiny fragments obtained as biopsy specimens in 3 cases. The number of silver-staining nucleolar organizer regions (AgNOR) per nucleus and MIB-1-positive rate were significantly higher in low-grade intraosseous osteosarcoma than in fibrous dysplasia, offering an advantage in differential diagnosis. Three patients (38%) developed high-grade sarcoma at the site of local recurrence after multiple intralesional excisions, and one of them died of the disease. The other 5 patients had a good clinical course after surgery with a wide margin. These findings indicate that preoperative diagnosis with radiologic investigation, including magnetic resonance (MR) imaging and histologic examination of biopsy specimens is essential in preparation for surgery with a wide margin, assuring a good clinical course, and the results of AgNOR and immunohistochemical MIB-1 staining might be helpful in differentiating low-grade intraosseous osteosarcoma from fibrous dysplasia.