Bisphosphonates are potent inhibitors of bone resorption and are commonly used in the treatment of bone metastases. In addition, they seem to influence cytokine secretion. Since the efficacy of IL-2 cancer immunotherapy, in part, depends on endogenous cytokine secretion, bisphosphonates could be effective in modulating IL-2 activity. High pretreatment levels of IL-6 seem to correlate with resistance to IL-2. On this basis, a pilot study was performed to evaluate the in vivo effects of the bisphosphonate, pamidronate, on blood levels of IL-6. The study included 7 patients with bone metastases due to solid tumours. Pamidronate was injected intravenously at 60 mg over 3 h. Venous blood samples were drawn before, at 1-h intervals during pamidronate infusion, then after 1 and 3 days. Mean serum levels of IL-6 significantly decreased during pamidronate infusion, then after 1 and 3 days, IL-6 mean levels still remained lower than control level, but differences were not significant. This preliminary study shows that pamidronate infusion induces a rapid but transient decline in IL-6 blood concentrations, and suggests a possible use of bisphosphonates to modulate the efficacy of IL-2 cancer immunotherapy.