Serial serum Carcinoembryonic antigen (CEA) levels were measured in 150 individuals (50 patients with breast cancer, 50 benign breast diseases and 50 other controls). These levels were correlated with clinicopathological parameters and follow-up information. Serum CEA levels were independent of the primary tumor status, their histology, lymphoreticular response and the patients' characteristics as well as the age, sex and the menstrual status. However, the nodal status, number of involved nodes and the grade of the tumors had significant influence on the level of serum CEA. Breast cancer patients especially those with metastasis had significantly higher serum CEA levels as compared to the controls and those with localised disease, irrespective of the site of metastasis. These levels were lowered appreciably by the disease regression and were raised or stable during the disease progression. Receiver operating characteristic (ROC) curve showed metastasis to be more frequent in patients with pretreatment serum CEA levels above 25 ng/ml and persistent post treatment CEA levels above 15 ng/ml. Serum CEA level was found to be a valuable prognostic indicator for advanced breast cancer and serial serum CEA levels provided an average lead time of about 3.9 months before the clinical appearance of metastasis.