The histological variability of solitary fibrous tumors may contribute to the difficulty in diagnosing these neoplasms, especially when they arise in extrathoracic sites. Like intrathoracic lesions, the behavior of extrathoracic solitary fibrous tumors is currently unpredictable because these types of tumor have only recently been recognized. This study therefore was undertaken to examine the clinical behavior and histological, immunohistochemical, and ultrastructural features of 24 extrathoracic solitary fibrous tumors with long-term follow-up. The patients comprised 10 men and 14 women, between 30 and 85 years of age (mean, 51 years). Ten tumors were located in the retroperitoneum or pelvis, 5 in the trunk, 4 in the extremities, 2 in the orbital region, and 1 each in the kidney, uterine cervix, and meninges. All of the tumors showed a classic morphological appearance, diffuse and strong immunoreactivity for both vimentin and CD34, and variable reactivity for bcl-2. All 7 cases examined ultrastructurally contained fibroblasts and myofibroblasts. Six tumors contained multinucleated giant cells, and in 4 cases these lined pseudovascular spaces with mononuclear cells, thus resembling giant cell angiofibroma and giant cell fibroblastoma. Other potentially similar spindle cell neoplasms mixed with adipose tissue, such as dendritic fibromyxolipoma, lipomatous hemangiopericytoma, cellular angiofibroma, and spindle cell lipoma, were considered in the differential diagnosis. One tumor displayed atypical histological features in the form of increased cellularity and nuclear pleomorphism, but this patient has remained free of disease for 14 years. Another 2 patients developed local recurrences at 6 months and 5 years, and a further patient developed pulmonary metastases that were diagnosed after 7 years. These tumors lacked any atypical histological features in the primary lesions. No patient has so far died of the disease. In conclusion, most extrathoracic solitary fibrous tumors appear to pursue a benign course, although, because some have the potential to recur or metastasize, careful long-term follow-up is necessary for all patients.