Solitary fibrous tumors (SFTs), rare in extrapleural sites, can present difficulties in diagnosis at these locations. From the files of the M. D. Anderson Cancer Center, we accessed 24 cases of extrapleural SFT (14 females, 10 males, ages 19 to 85 yr) obtained for clinical, histologic, immunohistochemical, and follow-up findings. Tumor locations included the head and neck (n = 12), the abdomen (n = 10), and the extremities (n = 2). Tumors were 2 to 25 cm in greatest dimension (mean, 8.2 cm) and were well circumscribed or encapsulated. Histologic features were typical of their pleural counterparts, e.g., bland spindle cells with some hypercellular areas and myxoid to hyalinized backgrounds. A hemangiopericytic vascular pattern was present in 19 cases and prominent in 11. Mitotic activity ranged from 0 to 4 counts per 10 high-power fields. Necrosis was seen in two cases. Focally increased cellularity was seen in seven cases and margins were positive in another seven cases. Spindle cells were positive for vimentin (19 of 19) and CD34 (18 of 20), and negative for cytokeratin (0 of 19). Fibroblastic differentiation was present in the three cases studied by electron microscopy. Flow cytometry in three cases revealed diploid cell populations. Follow-up for 19 cases (9 to 99 mo) revealed no evidence of metastasis, although one patient had residual tumor after an incomplete excision, and one patient died of unrelated causes. Histologic findings such as mitotic counts, necrosis, cellularity, and marginal status were not associated with outcome. We conclude that extrapleural SFTs are clinically and histologically similar to their pleural counterparts. Although the length of clinical follow-up was short (mean follow-up, 41.4 mo), recognizing these lesions is important because they typically follow an indolent clinical course if completely excised. Although CD34 is nonspecific, it is usually positive in SFTs and may aid in their diagnosis.