We report 65 cases of a hitherto undescribed neoplasm that occurs chiefly in children and young adults, and has morphologic features reminiscent of both a fibrous histiocytoma and fibromatosis. The median age of the 65 patients was 14.5 years; two-thirds (67.7%) of the patients were younger than 20 years. The lesion was more common in female patients (46 cases) than in male patients (19 cases). It usually presented as a slow-growing, poorly demarcated dermal or subcutaneous mass that rarely exceeded 3 cm in greatest diameter. Its most common location was the upper extremity (63.1%), especially the regions of shoulder and forearm. Under the microscope, the lesions were characterized by a multinodular or plexiform proliferation of histiocyte- and fibroblast-like cells associated with multinuclear giant cells. Differential diagnosis chiefly includes cutaneous fibrous histiocytoma, plexiform neurofibroma, fibromatosis, and benign and malignant giant cell tumor. Twenty of the 32 cases (62.5%) with follow-up information were alive and well after local excision, but the tumor recurred in 12 cases (37.5%). In two patients with recurrence, the disease metastasized to regional lymph nodes 9 and 36 months after the initial excision, respectively. Metastasis to the lung or other organs was not observed. We were unable to demonstrate a close correlation between biologic behavior and any specific clinical or morphologic parameter.