CA 15-3 (also known as MUC1) is the most widely used serum marker in breast cancer. MUC1 is a large transmembrane glycoprotein which is frequently overexpressed and aberrantly glycosylated in cancer. Physiologically, MUC1 appears to play a role in cell adhesion and the high levels present in cancer may be causally involved in metastasis. At present the main uses of CA 15-3 are in preclinically detecting recurrent breast cancer and monitoring the treatment of patients with advanced breast cancer. In a prospective study of 368 patients we show that patients with high preoperative levels of CA 15-3 (>30.4 U/mL) had a worse outcome than patients with low levels of the marker. In multivariate analysis CA 15-3 as a prognostic marker was independent of both tumor size and nodal status. Furthermore, in multivariate analysis the prognostic impact of CA 15-3 was stronger than that of tumor size and at least as strong as nodal status. CA 15-3 may thus be the first independent prognostic serum marker in breast cancer.