This multinational randomized double-blind trial, the Alendronate Phase III Osteoporosis Treatment Study, examined the results of alendronate therapy in postmenopausal women with osteoporosis who now have been followed for 10 years. A total of 227 women completed all phases of the study. Those participating were assigned to receive 5 or 10 mg of alendronate orally or placebo each day. The primary end point was the change in bone mineral density (BMD) in the lumbar spine. BMD also was measured at the femoral neck, trochanter, total proximal femur, forearm region, and the total body. From 17% to 31% of women in the various treatment groups had preexisting vertebral fractures. BMD continued to increase in the later years of the study in women given 5 or 10 mg of alendronate daily. The mean cumulative increase after 10 years in women taking 10 mg daily was 13.7%. Markers of bone remodeling (urinary N-telopeptides of type I collagen, bone-specific alkaline phosphatase) decreased, and this effect continued through 10 years of treatment. These markers increased within a year after alendronate was discontinued, although mean levels remained below baseline. There were no significant group differences in new vertebral fractures. The 3 groups also had similar safety profiles in the last 3 years of the study. No deaths were ascribed to alendronate therapy. Alendronate therapy continued to be effective over 10 years in these postmenopausal women, as estimated by both BMD measurements and effects on bone remodeling. The antifracture effect of alendronate did not appear to decrease over time.