The levels of carcinoembryonal antigen (CEA) were followed up for three years in 66 patients with breast cancer. In the group of patients treated surgically (N = 36, T1-2, NO-2, MO) the values of CEA were in agreement with the clinical course in 97% of the patients (35 out of 36 patients). Recurrence and/or dissemination of the disease were signalled by a rise in the serum CEA level to more than 15 micrograms/1 out four out of five patients more than 3-10 months before clinical manifestation. In the group of patients without surgery, who were treated by radiation and/or chemotherapy (N = 30, T2-4, NO-3, MO) the serum CEA values were in agreement with the clinical course in 71% of the patients (20/28). Within three years recurrence and/or dissemination were detected in 57% patients (16/28), ten patients died (10/30--two of them were not evaluated). The recurrence or dissemination were signalled in seven out of 16 patients (44%) more than 4-14 months before clinical detection, in three patients (19%) the rise was detected simultaneously with the clinical finding (examinations at 3-4 months intervals), and in 6 patients (38%) the CEA level was at variance with the clinical finding.