Vitamin D and calcium are essential in the treatment and prevention of osteoporosis. What dosage of vitamin D which is the ideal one, is not yet clear. We want to test the hypothesis that high dosage of vitamin D (i.e. 6500 IU/d) is better than standard dosage (800 IU/d) in a randomized double-blind trial. We will include 400 postmenopausal otherwise healthy women with T-score <= -2.0 in L2-4 or mean total hip. Everybody will receive calcium 1000 mg and vitamin D 800 IU every day. Half of the group will also receive vitamin D 40 000 IU/week, while the other half will have placebo. The study period is one year.