Dedifferentiation is a well recognized, if sometimes controversial, form of tumor progression in certain types of soft tissue and bone sarcoma, and confers a worse prognosis when compared with the low-grade counterpart. To date, dedifferentiation has not been described in solitary fibrous tumor (SFT). Among 948 cases of both intrathoracic and extrathoracic SFTs in our files accessioned between 1988 and 2008, we identified 8 cases of conventional SFT with a discrete anaplastic component, which we believe represents dedifferentiation. These occurred in 3 men and 5 women, 40 to 76 years old (median 60 y), and measured 3.4 to 20.0 cm (median 8.5 cm). Two cases were intrathoracic, 2 were located in the deep soft tissue of thigh, and single cases were located in the omentum, scalp, retroperitoneum, and abdominal wall. In addition to typical features of benign-appearing SFT there was an abrupt transition to nondistinctive high-grade sarcoma in all cases. The latter included epithelioid, round cell, and/or spindle cell components with increased mitotic activity, necrosis, and cystic degeneration. By immunohistochemistry, 7 of 8 cases were CD34 positive in the usual SFT areas, whereas 5 showed loss of CD34 in the poorly differentiated component. Six of 7 cases stained for p53 and p16 showed either negative or scattered positive cells in well-differentiated SFT areas, in contrast to positive or stronger and more diffuse staining in the high-grade component. Follow-up information available in 7 patients ranged from 1 to 58 months (mean 24 mo). Three patients with the largest tumors (9.0, 17.0, and 20.0 cm) died of disease, whereas 3 patients whose tumors measured 8.0 cm or less were treated by surgical excision only, and show no evidence of disease but with only limited follow-up. One patient with an 11.5 cm intrathoracic tumor is alive with disease at 58 months after recurrence and metastasis. We describe, apparently for the first time, what seems, at least in our view, to be dedifferentiation in primary SFT. Our results demonstrate that dedifferentiation in SFT, comparable with that in other low grade/intermediate soft-tissue tumors, poses a higher risk of tumor recurrence and/or metastasis, most notably in large and deep-seated tumors. Similar to other dedifferentiated sarcomas, abrupt transition between low grade and high-grade areas is typically observed with loss of CD34 positivity. The p53 and p16 overexpression in the high-grade component is common as in other dedifferentiated lesions, perhaps pertaining to the underlying molecular mechanism.