Renal solitary fibrous tumors (SFTs) have been reported infrequently. We report a 76-year-old man with a left renal mass that had previously been shown radiographically to be stable, but was now growing. Grossly, the mass measured 12 cm, was poorly circumscribed, and invaded beyond the renal capsule. Approximately 10% of the neoplasm consisted of haphazardly arranged spindle cells admixed with dense collagenous bands, which is typical of benign SFT. However, the remainder of the mass was composed of pleomorphic, spindled sarcoma cells with frequent mitoses and foci of necrosis. Immunohistochemically, we observed CD34 labeling in the benign SFT component with loss of expression in the sarcomatous component, focal labeling for Bcl-2 protein in both areas, and absence of labeling for cytokeratin, renal cell carcinoma marker, S100 protein, CD117, and muscle markers in both areas. To our knowledge, this is the first reported case of malignant renal SFT, likely representing transformation from a histologically documented benign SFT component.