Cancer is one of the leading causes of death in the United States. There is known to be a great variation in cancer incidence with diet, as has been recently reviewed (Grobstein et al., 1982). Epidemiological data suggest that environmental, specifically nutritional, factors play a major role in the etiology of cancer at many different sites (Grobstein et al., 1982; Doll and Peto, 1981; Correa, 1981; Phillips, 1975). There are now many different epidemiological studies which suggest that components of vegetables, particularly legumes (Correa, 1981), might play a beneficial role in lowering the incidence of cancer (some examples of such studies are given in the references cited above). Legumes are known to contain high levels of protease inhibitors (Birk, 1974). Rice, maize, and beans, all of which are known to contain high levels of protease inhibitors (Birk, 1974, 1975), in the diet are associated with a reduced incidence of colon, breast, and prostate cancers (Correa, 1981). The intake of breads and cereals, food sources which are also known to contain high levels of protease inhibitors, has been associated with a lowered incidence of oral and pharyngeal cancers (Winn et al., 1984). Other studies with high levels of protease inhibitors in the diet have also suggested a reduced risk of developing colorectal and breast cancers (Blondell, 1988). The role of vegetable-derived protease inhibitors as active cancer chemopreventive agents is discussed in several other chapters in this volume (e.g., Chapter 1).