Serial CEA determination have been performed in 335 patients with operable breast cancer who received radiotherapy and then were the subjects of a long-term follow-up study. Tumor extension was staged by the surgeon according to the TNM classification. Elevated pretreatment CEA levels (greater than 10 ng/ml) indicated metastatic spread even when this was not evident from the original TNM classification. Elevated CEA levels of greater than 4 ng/ml also led to a reevaluation of patients and in 20% metastatic spread was found. Therapy was adapted when patients had demonstrable distant spread. Response to treatment could be correlated with decreasing CEA levels while increasing CEA levels were generally found when disease progression was observed. During long-term CEA follow-up, 80% of recurrent cancers were signaled by increasing CEA levels. A mean lead time of 4.8 months was calculated for the initial CEA increase before clinical confirmation. Slope analysis of the posttreatment CEA time course represented a numerical parameter which was characteristic for osseous and/or liver metastases when values of greater than 0.5 ng/ml/10 days were recorded. Soft tissue, lymph node, lung and brain metastases showed generally a slope value of less than 0.5 ng/ml/10 days.