The role of blood tumor markers in monitoring response in advanced breast cancer is established in endocrine therapy and standard chemotherapy. This study examines marker levels in patients receiving new chemotherapy regimens. Thirty patients were recruited into two multicenter trials in which docetaxel-based regimens were used in 15 patients. The other 15 received doxorubicin-based regimens. Biochemical response calculated from a score using CA15.3, CEA and ESR was compared with UICC response. Marker changes at 2, 4 and 5 months correlated with UICC response at 3, 4(1/2) and 6 months, respectively (p < 0.03). Eleven patients achieved both clinical/radiological and biochemical response at the end of treatment; markers had not yet returned to below cutoffs in seven, suggesting a possible advantage to continue chemotherapy. No patient showed a biochemical response whilst judged clinically/radiologically progressive. Nineteen patients had progressed either clinically/radiologically or biochemically at six months; of these, eight showed progression assessed earlier by markers so that a median of four cycles of chemotherapy could have been saved. Measurements of serum c-erbB2 showed a correlation with tissue c-erbB2 staining in the primary tumor (p < 0.003). Among the patients with positive tissue staining, sequential changes in serum c-erbB2 completely paralleled initial response.