To assess the change in concentrations of circulating gastrointestinal cancer-associated antigens in response to therapy, we analyzed the sera of patients with hepatic metastasis from colorectal carcinoma who were treated with intrahepatic arterial chemotherapy. Serial serum samples were assessed for the tumor-associated antigens, carcinoembryonic antigen (CEA) and the gastrointestinal cancer antigen CA 19-9. Computed axial tomographic (CAT) scans were made to assess the size of the hepatic metastasis. In 9/10 of these patients the CEA predicted tumor response within 2-6 weeks after initiation of treatment, and in 7/10 the information was supported or more dramatically demonstrated by the CA 19-9. Combining data from both tumor markers may provide a more accurate assessment of the clinical response than one antigen alone. Recurrence of hepatic metastatic growth or extrahepatic tumor also was identified by elevation of one or both circulating tumor-associated antigens prior to other laboratory or clinical evidence of tumor growth.