A retrospective study was made of 41 patients treated for gastric myosarcoma to identify prognostic factors that influence results. The adjusted five- and ten-year survival rates were 56% and 43% respectively, with no significant difference between leiomyosarcoma and malignant leiomyoblastoma. A histopathologic grade of malignancy could be assigned to each tumor according to the degree of hypercellularity, nuclear abnormality, mitotic rate and other characteristics. High histopathologic grade, large tumor size (greater than 5 cm diameter) and invasion of adjacent organs adversely affected prognosis. Five-year survival after curative treatment was: 100% (9/9) for small tumors, of which six were treated by wedge gastric resection; 67% (8/12) for large tumors, mostly after subtotal gastrectomy; and 0% for tumors that invaded adjacent organs, despite extended resections. It is concluded that the management of gastric myosarcomas can be planned according to these prognostic factors and that multimodal therapy of tumors with adverse factors warrants consideration.