[Show abstract][Hide abstract]ABSTRACT: A solitary fibrous tumor (SFT) is an unusual spindle cell neoplasm that usually arises in the pleura but rarely occurs in the kidney. Despite its rarity, histological diagnosis of SFT is crucial to avoid misdiagnosis with other malignant tumors in the kidney. We report a SFT of the left kidney that presented as a malignant tumor on radiographic findings in a 75-year-old Japanese woman. The tumor was well circumscribed and composed of a mixture of spindle cells and dense collagenous bands with no areas of necrosis or cystic changes noted macroscopically or microscopically. Electron microscopy showed fibroblast-like cells with well-developed rough endoplasmic reticulum, surrounded with collagen fibers. Immunohistochemistry revealed reactivity for vimentin, CD34, Bcl-2, and CD99, but no staining for cytokeratin, S-100, desmin, actin, D2-40, or epithelial membrane antigen (EMA). These findings were compatible with those of SFT. Although SFT of the kidney is extremely rare, this tumor must be included in the differential diagnosis when we encounter renal tumors consisting of mesenchymal elements. Immunohistochemical study is the key to diagnosis for SFT, and ultrastructural study is useful for its diagnosis.