The individual and combined value of CA 15-3 and carcinoembryonic antigen (CEA) as breast cancer tumor markers was investigated in longitudinal studies. Patients included women at high risk for recurrence after primary therapy or undergoing treatment for metastatic disease. During follow-up, recurrent disease was documented in 33 of 39 (85%) patients including 11 with local recurrence and 22 with distant metastases. At the time recurrence was first documented by objective criteria 23 of 33 (70%) of the patients presented with abnormal CA 15-3 levels (greater than 36.7 U/ml) compared with 19/33 (58%) with abnormal CEA levels (5 ng/ml). Tumor marker elevations predominated in patients with advanced disease indicating that CA 15-3 and CEA are not reliable for the detection of early breast cancer. Both markers were helpful in monitoring therapeutic response since antigen levels correlated closely with disease status.