Although solitary fibrous tumors are well-recognized in the pleura, their occurrence in the paranasal sinuses is decidedly uncommon. We have encountered two cases of solitary fibrous tumors in the paranasal sinuses and report the clinicopathological findings including CD34 immunoreactivity. One tumor arose in a 55-year-old Japanese businessman and the other in a 53-year-old man who had been in the hospital for schizophrenia for 20 years. The tumors showed characteristic findings. Immunoperoxidase stains on paraffin sections showed staining of the cells for anti-vimentin, but there was no staining for anti-keratin, anti-S-100 protein, anti-desmin, anti-glial fibrillary acidic protein (GFAP), or anti-actin. Anti-CD34 monoclonal antibodies also reacted with these tumors, as those of the pleura generally do, and were found to be useful in diagnosing these tumors. CD34 immunoreactivity . Fukunaga et al.  reported that CD34 immunoreactivity presented in a solitary fibrous tumor of the nasal cavity, but separate tumors of the paranasal sinuses have not been analyzed. We have recently encountered two cases of solitary fibrous tumors of the paranasal sinuses. In this report, the clinicopathological features of these tumors of and their CD34 immunoreactivity were analyzed.