Zoledronic acid is a bisphosphonate, commonly used to treat bone diseases such as osteoporosis and cancer induced bone disease. Patients exhibit a variable sensitivity to zoledronic acid and the underlying explanation for this remains unclear. The objective of this study was to obtain more knowledge in this regard. We hypothesized that osteoclasts generated from different individuals will show a variable sensitivity to zoledronic acid in vitro. Osteoclasts were generated using monocytes from 46 healthy female blood donors (40-66 years). Matured osteoclasts were reseeded onto bone slices pre-coated with different concentrations of zoledronic acid and IC50 values were determined based on total eroded bone surface after 3 days of resorption. The IC50 for inhibition of osteoclastic bone resorption varied from 0.06 to 12.57 µM zoledronic acid, thus a more than 200-fold difference in sensitivity to zoledronic acid among osteoclasts from different individuals was observed. Multiple linear regression analyses showed that the determined IC50 correlated with: 1) smoking status, 2) the average number of nuclei per osteoclast in vitro. Further analyses showed that: 3) increasing protein levels of mature cathepsin K in osteoclast cultures rendered the osteoclasts less sensitive to zoledronic acid, 4) surprisingly, neither the gene nor the protein expression of farnesyl diphosphate synthase was found to correlate with the IC50, and 5) trench-forming osteoclasts were found to be more sensitive to zoledronic acid than pit-forming osteoclasts within the same cell culture. Thus, we conclude that there indeed is a high degree of variation in the potency of zoledronic acid on osteoclasts when generated from different individuals. We propose that these findings can explain some of the varying clinical efficacy of zoledronic acid therapy observed in patients. These findings may therefore be of clinical importance which should be investigated in a clinical trial combining in vitro and in vivo investigations.