The aim of this study was to evaluate the new serum tumor marker M3/M21 in comparison to CA 15-3, CEA and MCA in the follow-up of breast cancer patients. We investigated 267 serum samples from 35 patients suffering from invasive breast cancer, International Union Against Cancer (UICC) stage pT1 to pT4. Cut-off values for M3/M21, CA 15-3, CEA, and MCA were 25 U/I, 30 U/I, 5 ng/ml and 11 U/I, respectively. Serum tumor marker results were correlated with the results of the clinical and radiological examinations. The sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) of serum CA 15-3, CEA and MCA were 47/85/61/76%, 24/90/55/70% and 39/84/55/73%, respectively. Serum M3/M21 showed a sensitivity of 45%, specificity of 81%, PPV of 73% and NPV of 69%. The combination of CA 15-3 and M3/M21 increased the sensitivity to 55%, with a specificity, PPV and NPV of 83%, 79% and 86%, respectively. CA 15-3 measurements showed a lead time effect in 5 patients, ranging from 3 to 7 (median 4.1) months. The combination of CA 15-3 and M3/M21 provided lead time effects in 7 cases, ranging from 2 to 8 (median 5.7) months. We conclude that the detection of breast cancer recurrence with CA 15-3 is improved by combination with M3/M21.