Early data suggest that an annual i.v. infusion of zoledronic acid (ZOL) might have therapeutic use in women with osteoporosis. In this randomized, double-blind, double-dummy, multicenter, 24-week trial, we evaluated the onset of action of a single infusion of ZOL 5 mg (n = 69) compared with weekly oral alendronate (ALN) 70 mg (n = 59) in postmenopausal women with low bone mineral density (T score ≤ - 2 by DXA) as assessed by reductions in urine N-telopeptide of type I collagen (NTX) at week 1. The effects of these therapies on other markers of bone turnover, patient preference for once yearly i.v. vs. oral weekly treatment, and adverse events were also assessed. At week 1, ZOL 5 mg resulted in a significantly greater reduction in mean urine NTX from baseline than ALN 70 mg (P < 0.0001). Significantly greater reduction in urine NTX and serum β-C-telopeptide of type I collagen (β-CTX) were also observed in the ZOL 5 mg group at all post-baseline time points. Bone-specific alkaline phosphatase (BSAP) levels showed a more gradual reduction in both the ZOL 5 mg and ALN 70 mg groups, reaching premenopausal range by week 12. A comparable proportion of patients reported adverse events in each treatment group (ZOL 5 mg, 91.3%; ALN 70 mg, 86.4%). Transient, flu-like symptoms were the most common adverse events in the ZOL 5 mg group and resulted in a higher frequency of adverse events in this group during the first 3 days of treatment. After 3 days, adverse event rates were similar in the 2 groups. The majority of patients, including those experiencing flu-like symptoms, expressed a preference for annual i.v. therapy (66.4%) compared with weekly oral therapy (19.7%). We conclude that a single i.v. infusion of ZOL 5 mg reduced urine NTX levels more rapidly than weekly oral ALN 70 mg. The majority of study patients preferred an i.v. treatment regimen of ZOL 5 mg over weekly osteoporosis therapy with ALN 70 mg. © 2007 Elsevier Inc. All rights reserved.