Forty-four cases of leiomyosarcoma arising in the retroperitoneum (33 cases) and mesentery (11 cases) were analyzed clinicopathologically, and four were examined ultrastructurally. These tumors were more common in women (61%) than in men (39%), and the median age of the patients was 59 years. Except for three tumors, most were large and over 10 cm in the greatest diameter. Twenty-seven tumors were classical leiomyosarcomas, while seven were composed predominantly of epithelioid cells and classified as malignant leiomyoblastoma (epithelioid leiomyosarcoma). In addition to the classical microscopic features, dedifferentiated areas resembling malignant fibrous histiocytoma were noted in three cases. Ultrastructurally, the tumors were characterized by smooth muscle tumor cells showing a spectrum of differentiation of varying degrees. Of the 42 patients for whom follow-up information could be obtained, 33 (79%) died within a period of five years after the initial treatment. Mitotic activity showed no significant correlation with biological behavior; ten of the 15 patients in whom the mitotic counts were less than five per 10 high-power fields (HPF), expired of the tumor.