Human colon cancer usually develops on a mucosa which has already undergone multiple steps of genetic change. These multiple steps create a field effect characterized by the presence of morphologically normal, but biologically altered epithelial cells. This aims of this study were to evaluate whether the expression of carcinoembryonic antigen (CEA) can act as a phenotypic marker of the field effect, and to map its topography in relation to the presence of colorectal adenocarcinoma. The expression of CEA was tested by immunohistochemistry on morphologically normal mucosa at 4 increasing distances from 14 autologous cases of colorectal adenocarcinoma. CEA expression in the normal mucosa was compared to the tumor. The results show that in the mucosa adjacent to the edge of the autologous tumor, CEA is expressed to the same level as displayed in the carcinoma; there is a decrease in CEA expression in normal mucosa located at 1 cm or more from the edge of carcinoma. Mucosa sampled at 5 and 10 cm from the tumor expresses CEA at the same low level as in mucosa of control subjects with no colorectal neoplasm. In conclusion, this study demonstrates a gradient of CEA expression in the peritumoral area, supporting the concept of field effect, and maps its extent. These data are relevant to the biology of human colorectal cancer, and more practically, to the optimal location of surgical resection.