The aim of this meta-analysis was to evaluate the efficacy and safety of two bisphosphonates (alendronate and zoledronate) in the treatment of postmenopausal osteoporosis. The incidence of fractures was considered as primary endpoint. Only randomized trials with a follow-up period of 1 year or more were included in this systematic review and meta-analysis. We excluded studies that included patients with secondary osteoporosis especially in relation to therapy with corticosteroids or other drugs or diseases known to affect bone mineral density. Studies published as subgroup analysis, extension studies, economic evaluations, and comparisons with active control were excluded. The methodological quality of controlled clinical trials that met these inclusion criteria was evaluated. No studies were excluded from analysis due to lack of quality. The risk ratio of hip, vertebral and wrist fractures for alendronate were 0.61 [95% confidence interval (CI) 0.40-0.93], 0.54 (95% CI 0.44-0.66) and 0.65 (95% CI 0.33-1.25), respectively. Zoledronate risk ratio was 0.62 (95% CI 0.46-0.82) and 0.38 (95% CI 0.22-0.67) for hip and vertebral fractures, respectively.