Serum levels of carcinoembryonic antigen (CEA) and breast carcinoma antigen (CA 15.3) were determined in patients with breast carcinoma: in 129 before initial surgical or nonsurgical treatment and in 134 afterwards. Before any initial treatment, CEA was elevated in 15% of patients with Stage IV disease and CA 15.3 was high in 11% with Stage III and 48% with Stage IV. While monitoring management active disease was associated with elevated serum CEA in 66% of the patients, with elevated CA 15.3 in 73% and with at least one of the markers elevated in 86%. Both tests had high specificity (93% and 98%). The rise in serum CEA and, even more so, of serum CA 15.3 roughly paralleled the increase in bulk of the tumor: from locoregional disease through metastases to the lungs, bones, lungs with bones, and liver. Decreases in the levels of serum CEA and CA 15.3 reflected response to therapy, increases in the level of at least one marker-treatment failure, and levels fluctuating above the normal range indicated stationary disease. During follow-up, the predictive value of a negative test (levels within the normal range), suggesting that the patient might be free of disease, was 61% for CEA alone, 67% for CA 15.3 alone, and 80% for the two tests combined. We conclude that an elevated serum level of only one of the markers was useful for staging, implying advanced disease. Determination of both markers jointly was useful for monitoring the effectiveness of the therapy and for follow-up aimed at detection of relapse.