To 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 (greater than 25 units/ml) while 11% had an elevated CEA (greater than 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 (chi 2: 21.36, p less than 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.