Plasma carcinoembryonic antigen (CEA) levels were performed by radioimmunoassay in 234 patients with histologically proved breast cancer: 181 with advanced metastatic disease and 53 without distant metastases but nodal involvement at time of mastectomy. Four hundred and thirty-four assays were done and correlated with the clinical status of the patients. Values above 2.5 ng/ml were taken as abnormal. Active disease was associated with elevated plasma CEA levels. Very high values were not recorded in 109 patients when they were considered to be in complete remission, while only 22 patients out of 63 patients with progressive disease had normal values. In 16 of these values remained normal despite progression of disease. In 6 patients clinical relapse preceded CEA elevation by 2--5 months. Tumor burden and abnormal serial CEA values showed positive correlation in 38 patients. In 30 patients, change in clinical status and CEA values occurred simultaneously. In only 2 patients an increase in CEA value occurred 2--3 months before clinical documentation of relapse.