The objective of this study was to determine whether racial differences exist with regard to the anatomical distribution of adenocarcinoma of the colon. We conducted a retrospective study of consecutive patients with primary colon cancer seen during a 12-year period (1976 to 1987) identified by the Rush Tumor Registry. The setting was a large, tertiary-care, private medical center, located in the inner city of Chicago, Ill. A chi 2 unpaired Student's t test of association was performed to detect any statistically significant difference in the anatomical sites of distribution between blacks and whites. A statistically significant percentage of black patients had proximally located primary colon lesions compared with white patients. This observation was independent of the actual anatomical definition of right-sided or proximal colon lesions. These findings suggest that a significant percentage of black and white patients are less likely to be diagnosed with colon cancer at a more curable stage, when abiding by the current screening guidelines of the American Cancer Society and the National Cancer Institute. The impact is greater on black patients with adenocarcinoma of the colon.