Abstract： A 37-year-old female was admitted to the hospital for an itching and painful subcutaneous nodule with ulceration on the extensor aspect of her left forearm for more than 6 months.The pain was severe,continuous and localized.Systemic and local treatment with antibiotics resulted in no obvious improvement.The lesion had gradually increased in size over the past 6 months and the ulcer had enlarged for 1 month.On examination,a hard infiltrative plaque measuring about 5.5 cm × 4.0 cm with a well-defined margin was seen on the extensor aspect of her left forearm,along with ulceration and some dirty discharge on the surface.The diagnosis of fibrosarcoma,grade Ⅱ was eventually made by a biopsy of the lesion,which revealed increased pigmentation in the basal layer,and tumor tissue was tightly adherent to the epidermis.Dermis and subcutaneous fat layer were infiltrated with various sizes of spindle cells with fine collagen fiber bundles between the cells.Obvious atypia and mitotic figures were easily observed in some of the cells.Immunohistochemical analysis showed moderately positive staining for fibronectin,but negative staining for human melanoma black-45 (HMB45),S100,smooth muscle actin (SMA),Melan-a,high molecular weight cytokeratin (HCK),CD34,CD68 or cytokeratin.Some diseases should be differentiated from this case,including dermatofibrosarcoma protuberans,cutaneous spindle cell squamous carcinoma,atypical fibroxanthoma,malignant fibrous histiocytoma,and so on.