Elevated serum CA15‑3 assessed by enzyme‑linked immunosorbent assay (ELISA) has been considered a diagnostic marker of breast cancer. However, accumulating data indicate that the current ELISA system for detecting CA15‑3, which targets the peptide backbone of CA15‑3, is not sufficiently sensitive to detect early or localized breast cancer. In the present study, we designed an antibody‑lectin sandwich assay detecting glycosylation of CA15‑3 in patients with breast cancer. Ιmmobilized anti‑CA15‑3 monoclonal antibody captures CA15‑3 in serum, and glycosylation of the CA15‑3 is detected with Concanavalin A (ConA) lectin, which preferentially bind high‑mannose N‑glycans. ConA provided the best signal for detecting serum CA15‑3 among 9 types of lectin, Since CA15‑3 is a heavily glycosylated protein, detecting the glycosylation of CA15‑3 should be a much more sensitive way to assess CA15‑3 than the current ELISA method. Linear responses were obtained in the anti‑CA15‑3 antibody‑ConA sandwich assay when sera were diluted up to 2000‑fold. This dilution factor is comparable with that of the current ELISA system which allows 50‑ to 100‑fold serum dilutions. The glycosylation level of CA15‑3 was found to increase with increasing breast cancer stage in the sandwich assay. The assay system appeared to efficiently discriminate breast cancer stage I (sensitivity: 63%, specificity: 69%), IIA (sensitivity: 77%, specificity: 75%), IIB (sensitivity: 69%, specificity: 86%) and III (sensitivity: 80%, specificity: 65%) from benign breast disease. The antibody‑lectin sandwich assay shows promise as a new prospect for the early detection of breast cancer.