Solitary fibrous tumors (SFTs) often involve the pleura and also may encompass the peritoneum and nonserosal sites. On occasion SFTs mimics other neoplasms, including desmoplastic mesothelioma. CD-34, initially characterized as a hematopoietic progenitor cell antigen, recently has been identified in a small number of SFTs. Based on this observation, we compared the keratin, vimentin, and CD-34 expression of 19 SFTs and eight desmoplastic mesotheliomas. Fifteen of 19 SFTs (78.9%) expressed CD-34, whereas keratin expression was absent in all SFTs. In contrast, none of the desmoplastic mesotheliomas expressed CD-34 and keratin expression was found in seven of eight (87.5%). Vimentin expression was noted in 18 of 19 SFTs and in seven of eight desmoplastic mesotheliomas. We conclude that CD-34 expression distinguishes SFT from desmoplastic mesothelioma. Additionally, the results of our study support the idea that SFT is not derived from or related to conventional mesothelium.