This clinicopathologic study deals with 84 patients who had recurrent malignant fibrous histiocytoma (MFH), and is primarily concerned with 50 cases in which specimens of both the initial and recurrent tumors were available for histopathologic examination. Quantitative and qualitative differences in the histologic patterns characterizing MFH, such as storiform and myxoid, between the initial and recurrent tumors, were clear-cut in nearly two-thirds of the cases. Conversion in histologic subtypes of MFH occurred in nine cases, including six cases which converted from myxoid to nonmyxoid pleomorphic. The superficially located tumors recurred somewhat more frequently, but in such cases, there was a significantly better prognosis than was seen with the deeply located tumors. Changes in the degree of mitotic rate, nuclear atypism, and cellularity from the initial to the recurrent tumor correlated better with the biological behavior of the tumor than did the degree in the initial tumor itself. The increased proportion of histiocyte-like tumor cells in the recurrent tumor was another morpholic feature which indicated a much poorer prognosis than did the increased proportion of fibroblast-like tumor cells.