AbstractTo evaluate CA 15‐3, a new breast cancer associated antigen, and to compare it with carcinoembryonic antigen (CEA), all patients presenting with breast cancer had preoperative and serial (3‐monthly) postoperative levels measured. Of 124 patients with primary breast cancer, 23% had an elevated CA 15‐3 (>25 units/ml) while 11% had an elevated CEA (>5 ng/ml) (p=not significant). Neither marker was an indicator of spread to regional lymph nodes in primary breast cancer. In 45 recurrences of breast cancer, CA 15‐3 was elevated at the time of first recurrence in 58% while CEA was elevated in 47% (p=not significant). Of 17 patients with locoregional recurrence alone, none had a CA 15‐3 above 40 units/ml while 11 of 12 with synchronous locoregional and distant recurrence had a CA 15‐3 level greater than 40 units/ml (χ2∶ 21.36,p < 0.0001). This study shows that CA 15‐3, like CEA, is of little clinical value in primary breast cancer. CA 15‐3, however, is an accurate indicator (overall accuracy, 97%) of synchronous distant metastases in patients with locoregional recurrence from breast cancer. This information has important implications for further investigation and management of such patients.