Serial measurements of plasma carcinoembryonic antigen (CEA) levels were analyzed in 42 patients with advanced breast cancer undergoing systemic chemotherapy. Pretreatment CEA levels exceeded 5 ng/ml in 22 patients, and 19 of 22 serial assays uniformly heralded tumor regression as well as subsequent tumor relapse. A significant quantitative alteration in CEA levels was established as a minimum change of 20% within 8 weeks of therapy. In 13 of 15 patients responding to chemotherapy and in all patients with CEA levels higher than 35 ng/ml, this criterion was not abrogated, and there were no discordant observations. Rising CEA levels were correlated with subsequent progression of disease in all patients with elevated baseline levels at a minimum of 8 weeks before the progression was clinically evident. In advanced breast cancer the effectiveness of therapy and the development of tumor resistance may be monitored by serial plasma CEA levels, and specific quantitative criteria should be applied.