Inflammatory myofibroblastic tumor (IMT) or inflammatory pseudotumor is a spindle cell proliferation of disputed nosology, with a distinctive fibroinflammatory and even pseudosarcomatous appearance. Although the lung is the best known and most common site, inflammatory myofibroblastic tumor occurs in diverse extrapulmonary locations. We report our experience with 84 cases occurring in the soft tissues and viscera of 48 female patients and 36 male patients between the ages of 3 months and 46 years (mean, 9.7 years; median, 9 years). A mass, fever, weight loss, pain, and site-specific symptoms were the presenting complaints. Laboratory abnormalities included anemia, thrombocytosis, polyclonal hypergammaglobulinemia, and elevated erythrocyte sedimentation rate. Sites of involvement included abdomen, retroperitoneum, or pelvis (61 cases); head and neck, including upper respiratory tract (12 cases); trunk (8 cases); and extremities (3 cases). The lesions ranged in size from 1 to 17 cm (mean, 6.4; median, 6.0). Excision was performed in 69 cases. Eight had biopsy only. Five patients received chemotherapy or radiation in addition to undergoing biopsy or resection as initial treatment. Sixteen patients had multinodular masses involving one region. Clinical follow-up in 53 cases revealed that 44 patients were alive with no evidence of disease, four were alive with IMT, and five were dead. Thirteen patients had one or more recurrences at intervals of 1-24 months (mean, 6 months; median, 10 months). No distant metastases were documented. The five patients who died had complications either due to the location of the lesion (heart, peritoneum, retroperitoneum, or mesentery) or related to treatment (lymphoproliferative disorder following hepatic transplantation; sepsis following wound infection). The abdominal masses were the largest. All tumors were firm and white with infiltrative borders and focal myxoid change. Three basic histologic patterns were recognized: (a) myxoid, vascular, and inflammatory areas resembling nodular fasciitis; (b) compact spindle cells with intermingled inflammatory cells (lymphocytes, plasma cells, and eosinophils) resembling fibrous histiocytoma; and (c) dense plate-like collagen resembling a desmoid or scar. Immunohistochemistry demonstrated positivity for vimentin, muscle-specific actin, smooth muscle actin, and cytokeratin consistent with myofibroblasts. Based on this series, inflammatory myofibroblastic tumor is a benign, nonmetastasizing proliferation of myofibroblasts with a potential for recurrence and persistent local growth, similar in some respects to the fibromatoses.