From 1970 to 1991, 51 cases of smooth muscle tumors were diagnosed by fine needle aspiration biopsy. All were verified histologically and/or clinically, with no false-positive results. Of the 51 tumors, 41 were leiomyosarcoma and 10 leiomyoma. Among the 41 cases of leiomyosarcoma, 29 were the well-differentiated type; 8, poorly differentiated type; and 4, epithelioid type. Of the 10 cases of leiomyoma, 2 were the epithelioid type. The cytomorphologic features and cytologic patterns of various types of smooth muscle tumors observed in aspirate preparations are presented and compared in order to establish the cytologic criteria for differentiating malignant from benign smooth muscle tumors. Cytomorphologically the various types of smooth muscle tumors were different, and their cytologic features were sufficiently distinctive to distinguish one from the others. It appears possible to differentiate a well-differentiated leiomyosarcoma from a leiomyoma on the basis of cytologic findings observed in aspirate preparations. The recognition of different cytomorphologic features of various types of smooth muscle tumors is important in establishing an accurate cytologic diagnosis, which may be of practical significance to clinical management.