A rising CEA level is accepted as a sensitive indicator of recurrent colorectal carcinoma. In order to assess its efficacy in monitoring responses to chemotherapy in disseminated disease, 47 patients with documented metastatic colorectal cancer were studied. All had pretreatment CEA determinations and 40/47 (85%) patients had CEA levels greater than 4.0 ng/ml. Patients with hepatic metastases had the highest CEA values and none were found to have levels less than 4.0 ng/ml. Thirty patients received chemotherapy and had serial CEA determinations. 4/30 (13.3%) demonstrated probable tumor regression, 7/30 (23.3%) had stable disease, while 19/30 (63.3%) showed disease progression. CEA titers declined in all four responders, but in only one instance did the level fall to below 4.0 ng/ml and provide evidence of a tumor response not appreciated clinically. The only cytotoxic drugs effecting tumor regressions were 5-FU and 5-FUDR. CEA levels usually rose as disease progressed, but once elevated, absolute values did not correlate directly with tumor burden. The further usefulness of CEA assays in monitoring disseminated colorectal cancer awaits the development of improved chemotherapeutic agents.