The clinical significance of serum interleukin-6 (IL-6) and its correlation with cystatin C (Cyst C), an endogenous inhibitor of cysteine proteinase cathepsin K, was investigated by immunoassays in patients with bone metastasis from breast cancer (BCa) or prostate cancer (PCa). Additional studies were also performed in these patients to assess the effects of zoledronic acid (ZA) administration on the circulating levels of these molecules. Mean IL-6 and Cyst C serum concentrations were significantly increased in BCa patients and in patients with primary osteoporosis (PO) compared to healthy subjects (HS). However, Cyst C, but not IL-6, resulted significantly more elevated in BCa patients than in PO patients. Furthermore, in BCa patients no correlation was highlighted between IL-6 and Cyst C or between these molecules and some clinicobiological parameters of malignant progression. Mean IL-6 levels were also higher in PCa patients and in patients with benign prostatic hyperplasia (BPH) than in HS while Cyst C resulted significantly higher in PCa but not in BPH patients as compared to HS. In PCa patients, a positive correlation was highlighted between IL-6 and number of bone metastases or serum prostate-specific antigen but not with the Gleason score. Conversely, Cyst C levels did not correlate with any of the parameters considered above or with IL-6. Receiver operating characteristic (ROC) curve analysis showed a poor diagnostic accuracy of IL-6 and Cyst C to detect BCa patients with skeletal metastases while, in PCa patients, only IL-6 showed a fair diagnostic performance in this respect. Finally, the administration of ZA to patients with bone metastases induced a statistically significant increase of serum IL-6 and Cyst C only PCa patients with bone metastasis. These data indicate that IL-6 and Cyst C may be regarded as novel targets for cancer treatment and as markers of increased osteoblastic activity associated to bisphosphonate treatments in PCa patients with bone metastases.