This randomized, double-masked, placebo-controlled trial evaluated the safety, tolerability and effects on bone mineral density (BMD) of alendronate in a large, multinational population of postmenopausal women with low bone mass. At 153 centers in 34 countries, 1908 otherwise healthy, postmenopausal women with lumbar spine BMD 2 standard deviations or more below the premenopausal adult mean were randomly assigned to receive oral alendronate 10 mg (n = 950) or placebo (n = 958) once daily for 1 year. All patients received 500 mg elemental calcium daily. Baseline characteristics of patients in the two treatment groups were similar. At 12 months, mean increases in BMD were significantly (p</=0.001) greater in the alendronate than the placebo group by 4.9% (95% confidence interval 4.6% to 5.2%) at the lumbar spine, 2.4% (2.0% to 2.8%) at the femoral neck, 3.6% (3.2% to 4.1%) at the trochanter and 3.0% (2.6% to 3.4%) for the total hip. The incidence of nonvertebral fractures was significantly lower in the alendronate than the placebo group (19 vs 37 patients with fractures), representing a 47% risk reduction for nonvertebral fracture for alendronate-treated patients (95% confidence interval 10% to 70%; p = 0.021). Incidences of adverse events, including upper gastrointestinal adverse events, were similar in the two groups. Therefore, for postmenopausal women with low bone mass, alendronate is well tolerated and produces significant, progressive increases in BMD at the lumbar spine and hip in addition to significant reduction in the risk of nonvertebral fracture.