INTRODUCTION Different bisphosphonates have been shown to increase bone mineral density (BMD) and reduce the risk of fracture in osteoporotic patients. It is unclear how shifting from a treatment with one bisphosphonate to another will influence the evolution of BMD and bone turnover. METHODS In the present study, we followed BMD (DXA, Hologic QDR1000) of the lumbar spine (BMDL) and of the total hip (BMDH), bone alkaline phosphatase (Ostase, Hibritech), and urinary collagen cross links (pyridinoline, deoxypyridinoline, Biorad) in 39 patients treated with IV pamidronate (60 mg/3 months) since at least 2 years and who were shifted to oral alendronate (10 mg/day, n=18) or left to IV pamidronate (n=21) for 2 more years. RESULTS BMD increased similarly and significantly in both groups after 2 additional years of treatment as compared to baseline (P<0.05, sign test). BMDL: +3.8% in the alendronate group vs +4.1% in the pamidronate group; BMDH: +4.3% in alendronate group vs +3.6% in pamidronate group, There was no significant change in the biological parameters of bone turnover in any group. CONCLUSION The increase of BMD with both bisphosphonates in these previously treated patients was as expected after a 2 more years of treatment. Alendronate administration did not induce a larger gain in BMD as compared to cyclic pamidronate. Bone turnover was no longer affected by switching the bisphosphonate treatment.