Proliferative crypt changes have been noted in mucosa bordering colonic carcinomas, but their biological significance is disputed. We anticipated that zonal patterning of histological changes and cell cycle marker expression would provide clues to the pathogenesis of these border changes. 81 specimens were examined including carcinomas, adenomatours polyps, adenomas with early carcinoma, flat adenomas and aberrant crypt foci. The spatial distribution and frequency of micro-architectural features, and mucosal thickness were determined in a border domain of 150 300 sequential crypts/specimen. Immunocytochemical expression of Ki67 and p53 antigens in crypts also was semi-quantitatively examined. We found that in 100% of carcinomas two histologically abnormal zones (Proximate and Middle) separated tumor from normal mucosa. Differences in the feature frequency between zones were statistically significant (p<0.05). Both zones showed mild increases in crypt cell expression of Ki67, with a statistically significant relationship to zonal patterning (p<0.005). Weak expression of p53 only appeared in rare cells. Crypt elongation with mucosal thickening (1.9x normal, p<0.001) in the Proximate and Middle zones distinguished carcinomas from border changes in all benign lesions, except flat adenomas. Since this change occurs in all cases of carcinoma, there is no correlation with tumor stage or grade. Also in carcinomas, elaborate complexes of attached crypts (connected crypt structures) were characteristic of the Middle zone, so that proximate zone was always architecturally simpler. We conclude, that despite continuous carcinoma growth, the invaded border mucosa maintains a prototypical zonal organization of molecular and histological crypt changes This spatially organized reaction pattern is likely to reflect an interplay between regulated growth and destructive processes in response to advancing carcinoma. Compared to the edges of benign colonic tumors, the edges of carcinomas are distinctive and consistent enough to be diagnostically useful.