A serum assay of CA 549 (Hybri-BREScan CA 549 degrees, Hybritech), a new tumor marker, was performed in 129 patients with breast cancer and 35 healthy women, in parallel with CA 15.3 (ELSA-CA 15.3 degrees, CIS Biointernational). Comparing 95 women with primary breast carcinoma and 35 controls, Relative (or Receiver) Operating Characteristic (ROC) analysis revealed that the Area Under ROC Curve (AUC) of CA 15.3 was significantly higher than that of CA 549, indicating that, for our population, the first marker was more effective. Parallel and series analyses were also performed using ROC AUC and revealed that the combination of these two tests did not give more information than the CA 15.3 test alone; however, they did not in any way constitute diagnostic tools. In our experience, the best field of application for CA 549 seems to be the therapeutic monitoring and early detection of breast cancer recurrences. However, further investigations on a larger scale are necessary to assess more precisely the place of CA 549 in following the clinical course of breast cancer patients.