Arthur Purdy Stout and his co-workers, in several publications, raised two important issues concerning gut stromal tumors. First, they felt that all were of smooth muscle origin. Recent ultrastructural and immunohistochemical studies suggest that the component cells are basically undifferentiated, and there is only occasional emergence of smooth muscle features and, in some tumors, possibly features of other cell types as well, such as Schwann cells. Second, Stout felt that the high mitotic rate was the best predictor of malignancy, but he recognized that some tumors, even with low rates, could metastasize. Surprisingly, recent studies, even those covering large series, have done little to dispute these contentions. However, current data suggest that the diagnosis of malignancy can be made using multiple parameters, not all of which must be present in every sarcoma. These parameters include, in addition to mitotic rate, the size, gross invasion of adjacent organs, and cellularity, and all of these must be modified according to the location in the gut and the pattern of growth.