We report 38 cases of inflammatory fibrosarcoma occurring in 23 females and 15 males, 2 months to 74 years of age (median, 8.5 years; mean, 15 years) with symptoms of abdominal pain (17 cases), anemia (21 cases), fever (14 cases), mass (16 cases), and gastrointestinal obstruction (7 cases). Primary tumor sites included mesentery and retroperitoneum (31 cases), omentum (two cases), mediastinum (two cases), liver (one case), diaphragm (one case), and abdominal wall (one case). Sizes ranged from 2.4 cm to 20 cm (mean, 9.6 cm). Follow-up data in 27 cases revealed local recurrences in 10 patients, with multiple local recurrences in three and histologically proven distant metastases to lung (two cases) and brain (one case). Five patients died from their disease (median, 20 months). All tumors, including metastases, consisted of fibroblasts, myofibroblasts, and plasma cells, with variable degrees of fibrosis and calcification. Immunostains indicate myofibroblastic differentiation; 18 of 20 (90%) stained for actin, 15 of 18 (83%) for vimentin, and 10 of 13 (77%) for keratin (primarily in a submesothelial location). Ultrastructural studies also disclosed myofibroblastic features. The locally aggressive, recurrent nature of these neoplasms, as well as the occurrence of metastases and tumor deaths, indicate that they are potentially malignant neoplasms that we believe are better classified as inflammatory fibrosarcomas, not as cellular inflammatory pseudotumors.