Interleukin-6 (IL-6) is produced by renal cell carcinoma (RCC) cell lines and primary tumors. Using immunohistochemical staining in two RCC patients with hypercalcemia and high serum levels of free and total IL-6, we showed expression of IL-6 in metastatic bone tissue. The role of IL-6 in hypercalcemia and bone resorption would suggest that bisphosphonates or dexamethasone could be useful as adjuvant therapy for IL-6 dependent bone metastases which fail to respond to interferon alpha (IFN) alpha 2a and all trans retinoic acid (ATRA).