Background: Women with new vertebral fractures have an increased risk of back pain and functional limitation because of back pain. Alendronate sodium treatment reduces the risk of new vertebral fracture by 50% in postmenopausal women with osteoporosis. Objective: To determine the effect of alendronate therapy on days affected by back pain in postmenopausal women with existing vertebral fractures. Design: Three-year, placebo-controlled, randomized, double-blind study. Setting: Fifteen university-based research clinics in the United States. Participants: A total of 2027 postmenopausal women aged 55 to 81 years with low femoral neck bone density and a preexisting vertebral fracture. Intervention: Alendronate sodium (5 mg/d for 2 years and 10 mg/d for the third year) or placebo. Main Outcome Measures: Occurrence and severity of back pain, number of days with back pain, and number of days of bed rest or limited activity because of back pain during 3 years of follow- up. Results: Irrespective of treatment assignment, women with new, clinically recognized vertebral fractures during follow-up had an increased risk of days of bed disability and days of limited activity because of back pain after the fracture. Women receiving alendronate reported an average of 3.2 fewer days of bed rest (P = .001) and 11.4 fewer days of limited activity (not including days of bed rest) because of back pain (P = .04) during follow-up than those receiving placebo. In the alendronate group, relative to the placebo group, there was a reduced risk of 1 or more bed-rest days (relative risk, 0.68; 95% confidence interval, 0.53-0.87), of 7 or more bedrest days (0.44; 0.30-0.64), and of 7 or more limited-activity days (0.87; 0.76-0.99). There were no statistically significant differences between treatment groups in the frequency of days of back pain or increases in back-related disability between baseline and study end. Conclusion: In postmenopausal women with preexisting vertebral fracture, alendronate therapy for 3 years reduced the number of days of bed disability and days of limited activity caused by back pain.