Up to 80% of breast cancer patients developing metastases have high levels of CA 15.3. We studied the prognostic implications of CA 15.3 kinetics in 119 patients before and at first metastasis by univariate and multivariate statistics. At first metastasis, CA 15.3 was elevated in 82.4% of patients, with a lead time (median 162 days) in 42.0% of them. Kaplan-Meier analysis showed overall survival (median 1477 days) to be significantly related to estrogen receptor (ER) and progesterone receptor (PgR) status (p=0.0001) and tumor size (p=0.025). The interval between diagnosis and first abnormal CA 15.3 (p=0.0001), the CA 15.3 concentration (p=0.013), and the presence or absence of a lead time (p=0.001) also had prognostic value. ER and PgR status (p=0.0005 and p=0.0103, respectively), metastasis-free interval (p=0.0003), existence of a CA 15.3 lead time (p=0.0028), and days from diagnosis to first abnormal CA 15.3 (p=0.0055) entered in the Cox model. After first metastasis (median survival 573 days), ER and PgR status (p=0.0001 and p=0.0004, respectively), existence of a lead time for CA 15.3 (p=0.0138), and the concentration of first abnormal CA 15.3 (p=0.0145) had individual prognostic value. In the Cox model ER status (p=0.0001), nodal status (p=0.0191), existence of a lead time for CA 15.3 (p=0.0033), days from diagnosis to first abnormal CA 15.3 (p=0.0132), and concentration of first abnormal CA 15.3 (p=0.0320) were found to be independent prognostic variables. Compared to a matched historical control group that was not monitored by CA 15.3 assaying (n=140), the study group had a significantly longer survival after the first metastasis (p=0.0005). In conclusion, the kinetics of CA 15.3 before the first metastasis is of prognostic value. When associated with 18-fluorodeoxyglucose imaging, serial CA 15.3 assays may help to implement early treatment of metastases.