Zoledronic acid (ZOL) has shown beneficial effects on bone turnover and bone mineral density (BMD) in postmenopausal osteoporosis. This study compared the efficacy and safety of a once-yearly i.v. infusion of ZOL with weekly oral alendronate (ALN) in men with osteoporosis. In this multicenter, double-blind, active-controlled, parallel-group study, participants (n = 302) were randomized to receive either once-yearly ZOL 5 mg i.v. or weekly oral ALN 70 mg for 24 months. Changes in BMD and bone marker levels were assessed. ZOL increased BMD at the lumbar spine, total hip, femoral neck, and trochanter and was not inferior to ALN at 24 months [least squares mean estimates of the percentage increases in lumbar spine BMD of 6.1% and 6.2%; difference approximately 0.13; 95% confidence interval (CI) 1.12-0.85 in the ZOL and ALN groups, respectively]. At month 12, the median change from baseline of markers for bone resorption [serum β-C-terminal telopeptide of type I collagen (β-CTx) and urine N-terminal telopeptide of type I collagen (NTx)] and formation [serum N-terminal propeptide of type I collagen (P1NP) and serum bone-specific alkaline phosphatase (BSAP)] were comparable between ZOL and ALN groups. Most men preferred i.v. ZOL over oral ALN. The incidence of adverse events and serious adverse events was similar in the treatment groups. It is concluded that a once-yearly i.v. infusion of ZOL 5 mg increased bone density and decreased bone turnover markers similarly to once-weekly oral ALN 70 mg in men with low bone density.