Interleukin-18 (IL-18), a cytokine that plays an important role in the T-cell-helper type 1 response, acts as an angiogenesis and tumor suppressor. Intercellular adhesion molecule-1 (ICAM-1) has a potential role in immunoregulation by mediating immune cell infiltration into the tissue. The aim of this study was to evaluate IL-18 and soluble (s) ICAM-1 serum levels in breast cancer (BCa) patients with liver (BCaM1 h) or bone (BCaM1 b) metastases compared to BCa patients without metastases (BCaM0) and healthy donors (HDs). Furthermore, since IL-18 enhances ICAM-1 expression, we investigated whether there was a direct correlation between sICAM-1 and IL-18 serum levels. Serum IL-18 and sICAM-1 levels were assayed by immunoenzymatic methods. The serum sICAM-1 levels in the three groups of cancer patients were significantly higher (p<0.05) than those of HDs. Serum IL-18 levels were significantly higher (p<0.05) in BCaM1h and BCaM1b patients compared to BCaM0 patients and HDs. sICAM-1 proved to be closely correlated with serum IL-18 levels in HDs, whereas a weaker correlation was found in BCaM1h, BCaM1b and BCaM0 patients. The defective correlation between sICAM-1 and IL-18 found in cancer patients may contribute to our understanding of the immunity upset occurring in cancer. Our data suggest that IL-18, irrespective of its biological activity, could represent a marker for metastatic breast cancer.