The local expression of CD44 splice variants in human breast cancer tissue has been previously shown to be associated with metastasis. We show here that elevated systemic serum levels of CD44 splice variants occur in breast cancer and may represent a new tool for staging and differential diagnosis. Sera of node-negative and node-positive breast cancer patients in comparison with healthy control subjects were analyzed for serum CD44 (sCD44) and 2 different splice variants (v5 and v6). Node-positive breast cancer patients showed significantly (p < 0.01) elevated levels of sCD44-v5 and -v6 splice variants in comparison to node-negative patients and healthy controls. None of the node-negative breast cancer patients or healthy controls showed elevated levels of both sCD44-v5 and -v6. Interestingly, no differences were seen for serum levels of non-spliced sCD44-standard between the 3 groups. Soluble forms of CD44 variants may promote migration of tumor cells. This may occur through interference with tumor cell adhesion or by modulation of immune defense mechanisms.