OBJECTIVE： To explore the effects of alendronate sodium on bone mineral density （BMD） and bone marker in prostate cancer elderly patients after medical castration therapy. METHODS： In perspective study， 84 elderly patients undergoing medical castration therapy were selected and divided into treatment group （45 cases） and control group （39 cases） according to random number table. Control group received medical castration therapy+Calcium carbonate D3 tablets， 1 tablet， po，qn； treatment group was additionally given Alendronate sodium tablets 70 mg， po，once a week， 1 week after routine treatment， on the basis of control group. Treatment course of 2 groups lasted for 12 months. The levels of 25-OH-D， testosterone， BMD and bone marker were observed in 2 groups， and the occurrence of ADR was recorded. RESULTS： 3 cases of treatment group and 1 case of control group dropped out of the study. Before treatment， there was no statistical significance in above indexes between 2 groups （P＞0.05）. After treatment， 25-OH-D levels of 2 groups were increased slightly， but there was no statistical significance （P＞0.05）； testosterone level of 2 groups were decreased significantly compared to before treatment， with statistical significance （P＜0.05）. 12 months after treatment， BMD of lumbar vertebra， collum femoris and hip were increased significantly in treatment group， while those were decreased significantly in control group； there was statistical significance in above indexes between 2 groups （P＜0.05）. The levels of N-terminal midfragment of osteocalcin， procollagen Ⅰ N-terminal propeptide and β-crosslaps were decreased significantly in treatment group， while those were increased significantly in control group； there was statistical significance in above indexes between 2 groups （P＜0.05）. There was no statistical significance in the incidence of ADR between 2 groups（P＞0.05）. CONCLUSIONS： Alendronate sodium can prevent bone loss and reduce the rate of bone turnover in elderly patients with prostate cancer receiving medical castration therapy.