Zoledronic acid (ZA) delays the onset or reduces the incidence of skeletal complications in breast cancer patients with bone metastases. However, there are few data on the long-term renal safety of ZA. We retrospectively evaluated 43 breast cancer patients with bone metastases who received ZA more than 24 months. The following parameters measured prior to first ZA use and after the last dose of ZA administration were compared: serum creatinine (SCr), blood urea nitrogen (BUN), alkaline phosphatase (ALP), calcium (Ca), and phosphorous (P). Forty-three breast cancer patients with documented bone metastases were evaluated. Median age at the start of treatment was 53 years (range 37-77). Median overall duration of ZA administration was 36 months (25-62). There were no statistically significant differences in the pre- and post-treatment levels of SCr, BUN, Ca and P. However, ALP levels after long-term ZA administration were decreased significantly (P < 0.05). More than 24 months of ZA administration did not adversely affect the renal function. ZA can be used safely in breast cancer patients with bone metastases beyond 2 years.