Lipoma is a common benign soft tissue tumor. This article describes a massive osteolipoma, an unusual lipoma that is fixed to the femoral periosteum. A 21-year-old man presented with a subcutaneous mass at the knee region, which had been present for more than 36 months, and a slight limitation of joint flexibility. On physical examination, a mass approximately 12×6×2 cm(3) in dimension was palpable beneath the skin at the knee proximal medial area when the knee was in flexion. The mass was ovoid, hard, nontender, well demarcated, large, subcutaneous, and relatively fixed to the femur. Medical imaging examination showed that the femur had a well-demarcated mass with a basement. No prominent body weight loss was noted. The excisional mass of 16×12×10 cm(3) was not well encapsulated by a thin, fibrous membrane and had an apparently osseous basal portion. Intraoperative rapid frozen section revealed that the tumor was derived from the adipose cells. The postoperative course was uneventful. The definitive pathologic diagnosis was intermuscular osteolipoma without evidence of malignancy. No recurrence was observed at 6-month follow-up. Osteolipoma with independent bone and an osseous basal portion is rare, especially in young adults. Osteolipoma has the same prognosis as simple lipoma, and surgical excision is the recommended treatment. To the authors' knowledge, such a massive osteolipoma has not been reported. Copyright 2012, SLACK Incorporated.