Smooth muscle tumors of the esophagus (n = 29), stomach (n = 30), small intestine (n = 31) and large intestine (n = 12) were examined histologically and immunohistochemically for vimentin (Vim), desmin (Des), S100, fibronectin (FN) and cytokeratin (Cyt) and for nucleolar organizer regions (AgNOR). There were 49 cases of leiomyoma (LM), 51 cases of leiomyosarcoma (LS) and 2 cases of leiomyoblastoma(LB). Histologically, LS was characterized by cellular atypia, more cells in mitosis and invasion to adjacent normal tissue. No mitotic figure was observed in LM. AgNOR was more abundant in number in LS than LM (P < 0.001), but it had no direct relation to tumor size, number of cells in mitosis and patients' survival period. Immunohistochemical studies of Vim, Des, S100, FN and Cyt revealed that none of them was very helpful in the differential diagnosis of LM from LS.