We examined the effect of alendronate treatment for 3-4 yr on risk of new fracture among 3658 women with osteoporosis enrolled in the Fracture Intervention Trial. This cohort included women with existing vertebral fracture and those with osteoporosis as defined by T score of less than -2.5 at the femoral neck but without vertebral fracture. All analyses were prespecified in the data analysis plan. The magnitudes of reduction of fracture incidence with alendronate were similar in both groups. The two groups were, therefore, pooled to obtain a more precise estimate of the effect of alendronate on relative risk of fracture (relative risk, 95% confidence interval): hip (0.47, 0.26-0.79), radiographic vertebral (0.52, 0.42-0.66), clinical vertebral (0.55, 0.36-0.82), and all clinical fractures (0.70, 0.59-0.82). Reductions in risk of clinical fracture were statistically significant by 12 months into the trial. We conclude that reductions in fracture risk during treatment with alendronate are consistent in women with existing vertebral fractures and those without such fractures but with bone mineral density in the osteoporotic range. Furthermore, reduction in risk is evident early in the course of treatment. This pooled analysis provides a more precise estimate of the antifracture efficacy of alendronate in women with osteoporosis than that in prior reports.