Leiomyosarcoma of the sinonasal tract. A clinicopathologic study of nine cases. The clinicopathologic features of nine cases of sinonasal tract leiomyosarcoma (SNTL) referred to the Armed Forces Institute of Pathology, Washington, DC, during the period from 1970 through 1988 are described. This report represents the largest study to date on SNTL, and our results are compared with the 21 previously reported cases of SNTL. Of the nine cases described, patients ranged in age from 22 to 86 years (mean, 55 years). The most frequent clinical presentation was nasal obstruction unilaterally. The neoplasms were limited solely to the nasal cavity in four cases (44%) and involved both the nasal cavity and paranasal sinuses in the remaining five cases. Light microscopic, immunocytochemical, and ultrastructural features served to characterize these tumors as malignant neoplasms of smooth-muscle origin. In contrast with previous studies, immunocytochemistry was employed to differentiate SNTL from other spindle cell malignancies of the region, using newly available monoclonal antibodies to smooth-muscle antigens. Treatment was surgical. Radiotherapy or chemotherapy did not appear to affect the progression of the disease. Furthermore, no relationship was found between the aggressiveness of SNTL and morphologic parameters (eg, mitotic count and tumor size). Instead, prognosis was dependent on the distribution of disease at presentation. Of all 30 patients with SNTL described to date, 10 had the neoplasm confined solely to the nasal cavity. The 10 neoplasms did not recur. We conclude that SNTL may best be regarded as a locally aggressive neoplasm with only limited metastatic potential and that it could be curable by complete surgical excision. 