Maps of mortality rates in all U.S. counties for cancer of the stomach, colon, and rectum are presented. The maps show a strong geographic dependency indicating that environmental factors are important in the etiology of these cancers. Furthermore, the urban-rural differences which have been noted in the past are not as readily apparent in this study. Evaluation of all cancer sites demonstrates in general that the geographic dependency is highest for organs most exposed to the environment such as the organs along the alimentary canal and lowest for unexposed organs such as prostate, pancreas, and brain. By studying the correlation coefficients between mortality rates for cancer of different organ sites, they were grouped in subsets with high correlations for each pair in the same subset. Cancers of colon, rectum, and breast are shown to be very highly correlated in U.S. data as well as in international data.