This is a clinicopathologic study of 150 cases of fibrous histiocytoma of the orbit. The tumors occurred in adults (median age, 43 years). The upper and nasal portions of the orbit were the most common sites of involvement. The most common signs and symptoms were proptosis (60 per cent), mass (46 per cent), and decreased vision (25 per cent). Based on the histopathologic features, the tumors were classified in three groups: benign (94 cases), locally aggressive (39 cases), and malignant (17 cases). The biological behavior correlated well with the duration of symptoms, the size and margins of the mass, and the histologic classification. The rate of recurrence was 31 per cent for the benign tumors, 57 per cent for the locally aggressive tumors, and 64 per cent for the malignant tumors. Follow-up data were obtained for 123 patients, with a mean duration of seven years. The ten-year survival of patients with benign, locally aggressive, and malignant fibrous histiocytoma was 100 per cent, 92 per cent, and 23 per cent, respectively. Nine patients died as a result of the tumor, six from local invasion of adjacent structures and three from metastatic disease. Fibrous histiocytoma is the most common primary mesenchymal orbital tumor of adults. The origin of the neoplasm is probably a primitive mesenchymal cell. Complete surgical excision appears to be the treatment of choice.