Solitary fibrous tumor is a rare circumscribed neoplasm involving usually the pleura and now described in many organs and in the soft tissues with a favorable outcome when completely excised. We report herein a malignant solitary fibrous tumor of the pleura occurring in a 59-year-old woman. The tumor cells strongly expressed vimentin and CD 34. In addition a weak positive staining was obtained using CD 99 and bcl 2. A disconcerting focal immunoreactivity with cytokeratin was observed. Solitary fibrous tumor is composed of loosely arranged spindle cells of variable density and may occasionally mimic malignant mesothelioma, an hemangiopericytoma or synovial sarcoma. Immunohistochemical study is helpful demonstrating a strong positivity for vimentin and CD 34 and absence of staining for epithelial markers and S-100 protein.