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.

Inclusion Criteria:

• bone mineral density in L2-4 or mean total hip with T-score <= -2.0

Exclusion Criteria:

• current use of bisphosphonates, PTH-analogs, estrogen, SERM, p.o. steroids or use of any of these agents last year
• serum creatinin >110 umol/L
• systolic blood pressure >175 mmHg or diastolic blod pressure >105
• serious disease (heart failure, angina pectoris, myocardial infarction, diabetes, mental reduction, granulomatous disease like sarcoidosis, cancer)
• kidney stone
• serum calcium > 2.55 mmol/L
• suspect primary hyperparathyroidism with serum calcium >2.50 mmol/L combined with PTH > 5.0 mmol/L or serum calcium > 2.45 mmol/L combined with PTH >= 7.0 pmol/L