Annual intravenous administration of 5 mg of zoledronate decreases fracture risk over 3 years. The optimal dosing interval of 5 mg of zoledronate is not known. In order to determine the duration of the antiresorptive action of a single 5-mg dose of intravenous zoledronate, we conducted a 3-year double-blind, randomized, placebo-controlled trial in a volunteer sample of 50 postmenopausal women with osteopenia. The coprimary endpoints were the bone turnover markers β-C-terminal telopeptide of type I collagen (β-CTX) and serum procollagen type-I N-terminal propeptide (P1NP). Secondary endpoints were bone mineral density (BMD) at the lumbar spine, total hip, and total body. After 3 years, mean (95% confidence interval) levels of serum β-CTX and P1NP were 44% (27–60) and 40% (24%–56%) lower in the zoledronate group (p < .001 versus placebo for each marker). BMD was higher in the zoledronate group than in the placebo group by an average of 6.8% (4.6%–9.1%) at the lumbar spine, 4.0% (1.8%–6.3%) at the total hip, and 2.0% (0.9%–3.0%) at the total body (p < .001 for each skeletal site). Between-group differences in markers of bone turnover and BMD were stable from 12 to 36 months. These data demonstrate that the antiresorptive effects of a single 5-mg dose of zoledronate are sustained for 3 years; clinical trials to investigate the antifracture efficacy of dosing intervals longer than 1 year are justified. © 2010 American Society for Bone and Mineral Research.