Serial plasma CEA levels were determined over a period of 1-3 years in 114 patients receiving adjuvant chemotherapy for T1, 2 or 3a N+ MO breast cancer. CEA values were correlated with clinical status, scintiscans, and other biochemical parameters. CEA values greater than 2.5 ng/ml were considered abnormal. Forty-one patients had normal values throughout the adjuvant period. In 73 patients where abnormal values occurred, four different patterns were seen: (1) a statistically significant number (30/73) had initial elevations with a decreasing titer; (2) rising titer (10/73); (3) fluctuating titer of transient elevation (25/73); and (4) persistent elevation (8/73). Seventeen patients developed overt metastases, this was associated with a rising CEA in nine patients (P less than 0.002). CEA was more sensitive for predicting relapse than alkaline phosphatase or LDH. The correlation between CEA determination and the eventual development of metastatic disease is striking, and has implications for the design of future clinical trials. Results indicate that CEA could be used to identify high risk patients, to estimate efficacy of chemotherapeutic regime, and to determine optimal duration of therapy.