The carcinoembryonic antigen (CEA) test was studied in 54 patients with advanced stages of colon cancer which was treated with high doses of folinic acid + fluorouracil. The CEA test correlates evaluated included: prognostic value, performance status, metastatic pattern, histologic grading, predictive value for response to chemotherapy, and value differences in cases with partial response to therapy. CEA levels less than 5 ng/ml corresponded to a greater survival time than did levels greater than 5 ng/ml. A correlation of CEA with performance status and with metastatic pattern was demonstrated. A progressive increase in average CEA values corresponded to increases in neoplastic mass. Although CEA levels were not found to be an index for predicting the response to chemotherapy, there was a significant different between pre- and posttreatment levels for partial response. The results suggest that CEA offers an additional criterion for evaluating the response of colon cancer to chemotherapy and it also has a role in the staging of advanced disease.