In a placebo-controlled randomized clinical trial, zoledronic acid (4 mg via a 15-minute infusion every 3 weeks for 15 months) reduced the incidence of skeletal-related events (SREs) in men with hormone-refractory metastatic prostate cancer. Among 122 patients who completed a total of 24 months on study, fewer patients in the 4-mg zoledronic acid group than in the placebo group had at least one SRE (38% versus 49%, difference = -11.0%, 95% confidence interval [CI] = -20.2% to -1.3%; P =.028), and the annual incidence of SREs was 0.77 for the 4-mg zoledronic acid group versus 1.47 for the placebo group (P=.005). The median time to the first SRE was 488 days for the 4-mg zoledronic acid group versus 321 days for the placebo group (P =.009). Compared with placebo, 4 mg of zoledronic acid reduced the ongoing risk of SREs by 36% (risk ratio = 0.64, 95% CI = 0.485 to 0.845; P =.002). Patients in the 4-mg zoledronic acid group had a lower incidence of SREs than did patients in the placebo group, regardless of whether they had an SRE prior to entry in the study. Long-term treatment with 4 mg of zoledronic acid is safe and provides sustained clinical benefits for men with metastatic hormone-refractory prostate cancer.