Abstract: Natrium Fluoride (NF) is one of the best treatments of osteoporosis because it is cheap, it has no major side effects, and it is effective (Heany et al., Farley et al., Dure-Smith et al., Kanis et al., and Pak et al.). Our work is a prospective open study to demonstrate the efficacy of low dose NF (20 mg daily) in post menopausal osteoporosis. Fifty six patients with osteoporosis (t score less than 2.5 standard deviation, as defined by WHO) were selected as consecutive patients. The criteria for selection were: 1- No inflammatory disease such as rheumatoid arthritis. 2- No steroid or other medications known to decrease the BMD. 3- Patients not taking Hormone Replacement Therapy (HRT) for a valid reason. 4- Some patients having HRT, Received NF in addition to their HRT because of their very low t score (inferior to 60%). BMD was measured by Dual Energy X-R,y Absorptiometry (DEXA) on L1-L4 (lumbar spine) and the femoral neck before and after one year of treatment. NF was given in a slow release formula (20 mg tablet) once a day with meal. Calcium carbonate (1500 mg calcium element daily) was given as 500 mg tablets with 400 vitamin D3. Both NF tablets and Calcium D were formulated and manufactured by Darou Pakhsh Pharmaceutical Research Center. Result: In 77% of patients (CI=11) both spine and femoral neck responded favorably. In 20% either the spine or the femoral neck responded to the treatment. In 3% (CI=5) both sites lost bone despite the treatment. The mean spine BMD improved 3.6% and the mean femoral neck BMD 2.2% annually. In the responder group the mean improvement was 5.1% for the spine (86% of patients) and 3.3% for the femoral neck (88% of patients). No side effects were observed in these patients. Conclusion: Although NF was used in very low dose (half the dose used by Pak et al.) the results were approximately the same. Pak observed 4 to 6% annual increase for the spine and 2 to 4% for the femoral neck.