Cases of gastrointestinal smooth muscle tumor seen at M. D. Anderson Hospital and followed for a minimum of 10 years are presented. The tumors were classified as high-grade leiomyosarcoma (41 cases), low-grade leiomyosarcoma (13 cases), and leiomyoma (2 cases). All of the leiomyosarcomas originated in the stomach (21 cases), small intestine (29 cases), or rectum (4 cases) and appeared to have arisen from the muscularis propria. Leiomyosarcomas were considered high-grade when the maximal mitotic rate in ten consecutive high-power fields was ten or more and low-grade when this rate was lower (actual maximal rates in the low-grade group varied from 1-5/10 high-power fields). All patients with high-grade leiomyosarcoma died of tumor after intervals ranging from 5 to 90 months (median, 25 months). All but two with low-grade leiomyosarcoma also died of tumor, but frequently after much longer intervals (range, 42-221 months; median, 98 months; survival difference P = 0.002). Intervals to recurrence and metastasis were correspondingly longer in the low-grade group (up to 188 months). The two leiomyomas were small (less than 2 cm), had no mitotic figures, and were less cellular than any of the leiomyosarcomas. However, they both occurred in locations in which no leiomyosarcomas were seen (muscularis propria of the esophagus and muscularis mucosae of the rectum); therefore, the problem of distinguishing leiomyomas from leiomyosarcomas in sites where the latter did arise could not be resolved, particularly in view of the fact that fatal low-grade leiomyosarcomas had diameters as small as 1 cm and maximal mitotic rates as low as one per ten high-power fields. "Leiomyoblastoma" was not found to be an entity; it is recommended that this term be dropped.