Plasma CEA levels have been determined in 92 normal women and 768 women with benign or malignant breast diseases. Only one of 92 normal women had a CEA level above 5 ng/ml. Of 253 women with benign breast diseases (gross cystic disease, adenofibroma, fibrosis, etc.) only one had a CEA level above 5 ng/ml. Ninety-four percent of the above two groups of women had CEA levels below 3 ng/ml. Of 164 women operated upon for Columbia Clinical Classification Stage A or B breast carcinoma, preoperative CEA levels were above 5 ng/ml in seven (4%). Patients with a preoperative CEA level above 3 ng/ml seemed to have an increased incidence of tumor recurrence. Elevated CEA levels (greater than 10 ng/ml) in our postmastectomy population of 288 patients have correlated with development of metastases in 14 of 46 subjects. Of 216 patients under treatment for metastatic breast carcinoma, CEA levels above 10 ng/ml have been detected in 15 percent of patients with soft tissue metastases, 38% of patients with visceral metastases and 50% of patients with osseous metastases. Of metastatic breast carcinoma patients with CEA levels above 10 ng/ml serial measurements have correlated with the patients response to therapy, progressively increasing in treatment failures and decreasing in treatment responders.