Bisphosphonates are the treatment of choice for lytic bone lesions associated with breast cancer. In contrast, bone lesions associated with prostate cancer are predominately osteoblastic. Zoledonic acid (Zol) is a new-generation bisphosphonate that is approximately 2-3 orders of magnitude more potent than pamidronate (Pam) in preclinical models and has demonstrated clinical efficacy in patients with both lytic and blastic lesions. Zoledonic acid (4 mg via 15 min infusion) every 3-4 weeks was directly compared to Pam (90 mg via 2 hr infusion) in 767 patients with breast cancer and bone metastases. The primary endpoint was the proportion of patients experiencing a skeletal-related event (SRE) over 13 months. Zoledonic acid was as effective as Pam, and the proportion of Zol-treated patients with an SRE (42% in the hormonal therapy strata and 44% in the chemotherapy strata) was comparable to the original studies comparing Pam to placebo. Among 371 breast cancer patients receiving hormonal therapy, the proportion of patients with an SRE was 47% for Pam vs. 57% for placebo (P=0.057), and among 380 patients treated with chemotherapy, the proportions with an SRE were 43% for Pam vs. 56% for placebo (P=0.008) at 12 months. Zoledronic acid (4 mg) has been compared to placebo in a randomized Phase III trial involving 422 men with hormone-refractory prostate cancer metastatic to bone. Zoledonic acid demonstrated a significant advantage over placebo for median time to first SRE (median not reached for Zol vs. 321 days for placebo; P=0.011), the proportion of patients with an SRE over 15 months (33 vs. 44% for placebo; P=0.021), and mean skeletal morbidity rate (number of SREs/time, 0.08 vs. 1.49 for placebo; P=0.006). In addition, the effects of Zol were apparent early. At 3 months, only 12% of Zol-treated patients had an SRE vs. 23% for placebo (P=0.003), and at 6 months, the proportions were 21 vs. 31% for placebo (P=0.025). In contrast, a previous study of Pam in 236 prostate cancer patients found that Pam was no more effective than placebo in reducing bone pain or SREs over 6 months. In these studies, Zol was well tolerated with a safety profile similar to other IV bisphosphonates. In conclusion, Zol is the first bisphosphonate to demonstrate efficacy in both lytic and blastic disease. The unique properties of this novel agent should be further explored in future clinical trials.