Serum levels of ovarian carcinoma antigen (CA 125) and breast carcinoma antigen (CA 15.3) were determined in 237 patients with breast carcinoma, 121 before any therapy and 116 after initial treatment, during uneventful follow-up or at the time of relapse. The aim was to assess how often the CA 125 test failed, i.e., was false-negative in patients in whom the CA 15.3 test was true-positive and, more important, whether it gave diagnostic information in patients in whom the CA 15.3 test failed. Before surgery or other initial therapy, serum CA 125 and CA 15.3 gave similar information in 85.1 percent of the patients: true-positive in 4.1 percent and false negative in 81.0 percent: CA 125 gave less information in 13.2 percent; and more information in only 1.7 percent. During follow-up, serum CA 125 and CA 15.3 gave similar information in 73.3 percent of the patients: true-positive (i.e., rising persistently from a nadir or elevated above 65 U/ml) in 23.3 percent, true-negative in 36.2 percent, and false-negative in 13.8 percent; CA 125 gave less information in 25.0 percent: false negative in 22.4 percent and false-positive in 2.6 percent; and more information in only 1.7 percent. Therefore, the CA 125 test appears useless for staging and is redundant when the CA 15.3 test is employed, for management of patients with breast cancer.