This study was designed to investigate the value of tumor M2 pyruvate kinase (tumor M2-PK) determination as an early marker for response to trastuzumab therapy in patients with metastasized breast cancer. Plasma samples of 20 trastuzumab patients were collected immediately after standard hematological investigations. The tumor M2-PK level was quantified using an enzyme linked immunosorbent assay (ScheBo Biotech) and CA 15-3 was measured automatically using the Bayer Immuno 1 immunoanalyzer and the corresponding assay. Each assay was performed in duplicate. The values were analyzed for correlation to the clinical course of each patient. Median observation time was 13 months with a range from 4 to 22 months. In 17/20 (85%) patients, tumor M2-PK determination was a marker for the clinical course of their disease. In this 'tumor M2-PK sensitive' group, 49 known clinical events (remission or progression according to UICC criteria) were recorded. The variation in tumor M2-PK level paralleled 63% of the clinical events (31/49). Our data suggest that tumor M2-PK determination in the plasma of patients with metastasized breast cancer could be a helpful tool for monitoring therapeutic success.