The distinction of solitary fibrous tumors (SFTs) from histologically similar neoplasms relies heavily on a characteristic microscopic appearance. No discriminating ultrastructural or immunohistochemical features are known. We evaluated 22 SFTs and 118 other tumors often considered in the differential diagnosis for immunoreactivity using a monoclonal antibody directed against the human hematopoietic progenitor cell antigen, CD34. All the SFTs (22 of 22, 100%) demonstrated strong CD34 immunoreactivity, irrespective of tumor site and histologic grade. Strong and generalized CD34 positivity was also found in most dermatofibrosarcoma protuberans (11 of 12, 92%) and occasional smooth-muscle tumors (leiomyomas 2 of 11, 18%; leiomyosarcomas 2 of 11, 18%). Variable numbers of CD34 positive cells were present in all neurofibromas (9 of 9, 100%) and focally present in most schwannomas (8 of 9, 89%). Some of the hemangiopericytomas (7 of 16, 44%) exhibited CD34 immunoreactivity, however, generally with weak intensity and patchy distribution. CD34 immunoreactivity was not observed in mesotheliomas (0 of 20, 0%), synovial sarcomas (0 of 13, 0%), fibrosarcomas (0 of 12, 0%), or spindle-cell thymomas (0 of 5, 0%). We conclude that CD34 immunoreactivity is a sensitive marker for SFT and, in conjunction with an appropriate immunohistochemical panel, may be useful in discriminating SFTs from other histologically similar neoplasms. The observation that some mesenchymal stromal cells and SFTs share a CD34-positive immunophenotype suggests a histogenetic relationship. The inclusion in this study of two cases of SFTs arising in the orbit establishes another site of origin for this tumor and provides further support for a mesenchymal histogenesis.