Eighty-three women with breast cancer (57 with systemic metastasis, 26 without) were investigated for serum hyaluronan (HA) and compared to 50 patients with benign diseases of the breast. Hyaluronan was significantly increased in sera of metastatic patients compared to sera of non-metastatic patients (p < 0.0001) and also in sera of non-metastatic patients when compared to control sera (p < 0.01). The difference was not related to the number of metastatic sites involved. Three months after starting cytotoxic chemotherapy in metastatic patients, lower HA concentrations were observed in patients responding to chemotherapy. The initial level of serum HA had no predictive value concerning response to chemotherapy.