The clinical and pathologic features of three new and seven previously described solitary fibrous tumors of the peritoneum are reviewed. The male-female ratio has been 7:3, and the patients have ranged from 27 to 64 (average, 54) years of age. Most of the tumors have been large (over 9 cm) and have been responsible for significant symptomatology. Microscopic examination has typically shown mildly to moderately cellular fibrous tumors with prominent hyalinization, although occasional tumors have exhibited focal marked cellularity. Cytologic atypicality and mitotic figures have been rare or absent. Immunohistochemical stains have shown positivity only for vimentin. There has been no evidence of recurrence in any of the cases. The clinical, gross, histologic, and immunohistochemical features of these tumors are identical to those of fibrous tumors of the pleura, and a similar origin from submesothelial mesenchyme is likely.