We describe 10 solitary fibrous tumors (SFT) with high-grade malignant overgrowth, all of which showed the presence of a synchronous or previous classic SFT/malignant SFT (MSFT) component. Seven were “dedifferentiated,” with an abrupt transition from a classic SFT/MSFT to a high-grade component consisting of a nondistinctive high-grade sarcoma in 4 cases and divergent differentiation in 3. The nondistinctive high-grade component consisted of epithelioid and/or spindle cells often associated with overt pleomorphism or small round cell sarcomas. The divergent differentiation featured a rhabdomyosarcoma in 2 cases and an osteosarcoma in 1. Three cases (tentatively called “evolved”) showed a gradual transition from classic SFT/MSFT to a nondistinctive high-grade sarcoma or presented features of high-grade sarcoma at the time of metastasis (assessed by fine-needle aspiration cytology) without any component suggesting a diagnosis of classic SFT/MSFT. The high-grade component showed loss of CD34 expression in half of the dedifferentiated SFTs and all of the dedifferentiated SFTs with divergent differentiation, whereas Ki-67 was markedly increased in all of the evaluable cases and paralleled the tumor grade. In 4 cases, the expression and phosphorylation status of key factors that control transcription and protein synthesis were also investigated. Both S6 and 4E-BP1 showed low activation in the low-grade MSFT and a high level of activation in the high-grade component. Seven of the 10 patients died of their disease during follow-up, with a median overall survival of 73 months (range, 5 to 288 mo). The median time to distant metastasis was 156 months after the initial diagnosis, and median overall survival from the first signs of metastasis was 8 months.